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Integrated correlation of the Vendian to Cambrian Arroyo delSoldado and Corumba Groups (Uruguay and Brazil):palaeogeographic, palaeoclimatic and palaeobiologic
implications
Claudio Gaucher a,*, Paulo Cesar Boggiani b, Peter Sprechmann a, AlcidesNobrega Sial c, Thomas Fairchild d
a Facultad de Ciencias, Departamento de Paleontologıa, Igua 4225, 11400 Montevideo, Uruguayb Universidade Federal de Mato Grosso do Sul (UFMS), Caixa Postal 549, Cidade Universitaria S/N, 79070-900 Campo Grande, MS,
Brazilc Departamento de Geologıa, Centro de Tecnologia e Geociencias, Universidade Federal de Pernambuco (UFPE), Caixa Postal 7852,
Recife PE-50732-970, Brazild Instituto de Geociencias, Universidade de Sao Paulo, Caixa Postal 11348, Sao Paulo SP 05422-970, Brazil
Received 30 May 2001; accepted 6 September 2002
Abstract
The Corumba Group of SW Brazil and the Arroyo del Soldado Group (ASG) of Uruguay are correlated on the basis
of litho-, bio- and chemostratigraphy. Both units represent marine sedimentation with alternating siliciclastics and
carbonates developed on a stable continental shelf. In the Corumba basin, sedimentation began in the Varangerian,
represented by the glaciomarine Puga Formation. A series of sea-level fluctuations coupled with climatic changes are
recorded up section. While uppermost deposits of the ASG are of lowermost Cambrian age, sedimentation ceased in the
latest Vendian in the Corumba basin. An assemblage of six species of organic-walled microfossils dominated by
Bavlinella faveolata and Soldadophycus bossii , three species of vendotaenids and two species of skeletal fossils (Cloudina
and Titanotheca ) is described from the Corumba Group. The vendotaenid Eoholynia corumbensis sp. nov is described
from siltstones of the Guaicurus Formation. An important diversity of skeletal fossils in the Corumba, Arroyo del
Soldado and Nama groups points to favourable Vendian palaeoclimatic conditions in SW-Gondwana. Preliminary
carbon isotopic data show a series of alternating positive and negative excursions, corroborating the upper Vendian age
indicated by fossils for both units. Previously reported strontium isotopic data are also consistent with this age. It is
postulated that the Corumba and ASGs were deposited onto the same shelf, which opened to the east. The Rio de la
Plata Superterrane (Craton) extends farther to the north than previously expected, or it was already amalgamated with
the Amazonian Craton by Vendian times. Collision of the platform with the Parana Block caused closure of the basin
during the Cambrian-Early Ordovician. Finally, models of Neoproterozoic glaciations based on enhanced
bioproductivity driven by high nutrient availability are discussed.
# 2002 Elsevier Science B.V. All rights reserved.
* Corresponding author.
Precambrian Research 120 (2003) 241�/278
www.elsevier.com/locate/precamres
0301-9268/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 1 4 0 - 7
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Keywords: Vendian; Neoproterozoic; Brazil; Uruguay; Palaeogeography
1. Introduction
The correlation of the Arroyo del Soldado
Group (ASG) of Uruguay with the Corumba
and Jacadigo Groups of southwestern Brazil
(Fig. 1) was first postulated by Gaucher et al.
(1996: 360�/361), on the basis of similar age,
similar depositional environments, probable con-
tinuity in subsurface, and existence of Bavlinella -
dominated acritarch-assemblages in these groups.
This working hypothesis proved to be a powerful
predictive tool, because it could explain many
observations made thereafter. Gaucher and
Sprechmann (1998, 1999) described a skeletal
assemblage from the Yerbal Formation of the
ASG containing Cloudina , which is a common
fossil in the limestones of the Tamengo Formation
of the Corumba Group (Hahn and Pflug, 1985;
Zaine and Fairchild, 1985; Boggiani et al., 1993;
Boggiani, 1998; Gaucher 1999). Boggiani (1998)
presented sedimentologic, carbon and strontium
isotopic data for the Corumba and ASG, con-
sidering that a correlation of both units is possible.
Gaucher (1999, 2000) compares the fossil content,
lithostratigraphy, chemostratigraphy and sedimen-
tary environments of the Arroyo del Soldado,
Jacadigo and Corumba Groups. The author con-
cludes that while the Arroyo del Soldado and
Corumba Groups are correlative and of Red-
kinian�/Kotlinian (upper Vendian) age, the Jaca-
digo Group is composed of glacial deposits and
BIF’s of the Rapitan-type, probably generated
during the Varangerian glaciation, as also postu-
lated by Almeida (1984), Walde (1987: 103),
Alvarenga (1990) and Boggiani (1998). In this
paper, we present in some detail the evidence that
supports the correlation of these important Ven-
dian to Cambrian units of South America. Newly
discovered microfossils from the Corumba Group
and carbon isotopic profiles of the ASG are
described as well. Finally, the paper deals with
the consequences of this new data for the poorly-
known palaeogeography of the Rıo de la Plata
Fig. 1. Map of SE-South America showing outcrop areas of the
Corumba and ASGs and major tectonostratigraphic units.
Boundary of post-Cambrian sedimentary cover is indicated as
well. PAT, Piedra Alta Terrane; NPT, Nico Perez Terrane;
CDT, Cuchilla Dionisio Terrane; RAB, Rıo Apa Block.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278242
Page 3
Superterrane (Gaucher, 1999, 2000) during theVendian�/Cambrian (Unrug, 1996).
2. Geological setting
2.1. The Arroyo del Soldado Group
This lithostratigraphic unit was defined by
Gaucher et al. (1996), to include thick (�/5000m) marine shelf deposits, which occur in the Nico
Perez Terrane of Uruguay. The boundaries of the
Terrane also represent the boundaries of the
occurrences of the ASG (Fig. 2), namely: the
Sarandı del Yı-Piriapolis Shear Zone to the west,
the Sierra Ballena Shear Zone to the East and the
Cerro Partido Thrust to the south (Gaucher et al.,
1998b; Gaucher, 1999, 2000). To the north, theoccurrences of the ASG are covered by Phaner-
ozoic sedimentary units and volcanics of the
Parana Basin. The northernmost outcrops of the
Group have been recognized by Gaucher et al.
(1998c) and described by Gaucher (1999, 2000) in
the so-called Isla Cristalina de Rivera, near the
boundary with Brazil (Fig. 2). The unit lies with
erosional and angular unconformity on: (1) aMesoproterozoic metasedimentary complex infor-
mally known as Basal Group (Sprechmann et al.,
1994; Gaucher et al., 1996) or Grenvillian Meta-
morphic Complex (Bossi et al., 1998), (2) Archean
(3.4�/2.7 Ga) metasedimentary units recently dis-
covered in the vicinity of Piraraja (Campal and
Schipilov, personal communication, 2000; Fig. 2);
(3) Palaeoproterozoic (17849/5 Ma: Campal andSchipilov, 1995) rapakivi granites and (4) other
granites of undetermined age. The basal Yerbal
Formation (Gaucher et al., 1998a; Gaucher and
Sprechmann, 1999; Gaucher, 2000) represents a
siliciclastic, deepening-upward sequence, and is
characterized by thin conglomerates and arkoses
at the base, passing into green siltstones and
banded siltstones at the top. Oxide-facies BIFhave been recently discovered at the top of this
unit near the city of Minas (Fig. 2). The Yerbal
Formation is concordantly overlain with sharp
contact by the Polanco Formation, which marks
the development of a large carbonate ramp in the
basin. The unit is characterized by bluish gray to
black limestone�/dolostone rhythmites, frequentcarbonatic tempestites, pure calcisiltites and dolo-
siltites, and rare oolitic calcarenites (Gaucher,
1999, 2000). In the shallowest areas of the basin
(to the west), carbonates of the Polanco Forma-
tion are concordantly overlain by conglomerates
and arkoses of the Barriga Negra Formation (Fig.
4), recording a major regression (Gaucher et al.,
1998a; Gaucher, 2000). This unit passes upwardsinto shales and siltstones of the basal Cerro
Espuelitas Formation. In the deepest sections (to
the E), the Polanco Formation shows a direct
transition into the Cerro Espuelitas Formation
(Gaucher, 1999, 2000; Fig. 4). The latter unit is
made up of an alternation of dark shales, thick
chert-deposits and oxide-facies BIF with up to 35
wt.% magnetite and/or hematite (Gaucher andSchipilov, 1994; Gaucher et al., 1996, 1998b;
Gaucher, 1999, 2000). The Cerro Espuelitas For-
mation is truncated by an erosive surface that
probably marks an important regression (Gau-
cher, 2000). The Cerros San Francisco Formation
was deposited above this surface, and is character-
ized by very mature quartz-arenites with wave and
current ripples, hummocky cross-stratification andlow-angle cross bedding (Montana and Sprech-
mann, 1993; Gaucher and Schipilov, 1993; Gau-
cher et al., 1996; Gaucher, 2000). Finally, the
Cerros San Francisco Formation passes upwards
into stromatolitic and oolitic limestones of the
Cerro Victoria Formation, which contains ichno-
fossils of probable Cambrian age (Montana and
Sprechmann, 1993; Gaucher, 1999, 2000).The ASG has been folded and intruded by
granites and syenites in the Cambrian, granitoids
yielding ages around 540�/510 Ma (Bossi and
Navarro, 1991: 342; Kawashita et al., 1999; Fig.
2). K�/Ar datings of illite-rich pelites belonging to
the Group yielded recrystallization ages between
5329/16 and 4929/14 Ma (Cingolani et al., 1990;
Gaucher, 2000), well in accordance with agesobtained for intrusive granites. Considering these
ages, the orogenic event that determined the
deformation of the ASG can be taken as a terminal
event of the Brasiliano Megacycle or, more
probably, represent a separate orogeny (Paraguay
Orogeny of Basei and Brito Neves, 1992). As
stated by Gaucher (2000), the latter interpretation
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 243
Page 4
seems more plausible because the orogeny records
the collision of a typical Brasiliano-block (Cuchilla
Dionisio Terrane) with a cratonized block, onto
which the Arroyo del Soldado-shelf developed.
2.2. The Corumba Group
With minor changes, the lithostratigraphical
scheme proposed by Almeida (1965) has been
Fig. 2. Geographic distribution of the ASG and other Neoproterozoic-Cambrian (meta)sedimentary units in Uruguay, and some
important Cambrian intrusive granites, slightly modified from Gaucher (2000). SYPSZ, Sarandı del Yı-Piriapolis Shear Zone; SBSZ,
Sierra Ballena Shear Zone; CPT, Cerro Partido Thrust; IPSZ, Isla Patrulla Shear Zone; IPB, Isla Patrulla Block; ICR, Isla Cristalina
de Rivera; ICA, Isla Cristalina de Acegua.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278244
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adopted by most authors recently dealing with theCorumba Group (Almeida, 1984; Zaine, 1991;
Boggiani et al., 1993; Boggiani, 1998; Alvarenga
et al., 2000). Boggiani (1998) includes the Cadiueus
Formation in the base of the Corumba Group,
after the recognition of a gradational contact
between the Cadiueus and Cerradinho Forma-
tions. The group rests with angular and erosional
unconformity on (1) granites and gneisses of theBasal Complex of the Rıo Apa Block, sensu
Correa et al. (1979), of undetermined age but
probably reworked in the Mesoproterozoic; and
(2) Mesoproterozoic (Uruacuan) biotite gneisses,
micaschists and quartzites of the Alto Terere
Metamorphic Complex (Correa et al., 1979; Al-
meida, 1965, 1984; Schobbenhaus Filho et al.,
1979). The Corumba Group overlies conformablythe Puga Formation at many localities, like at the
Morro do Puga (Correa et al., 1979; Boggiani,
1998; Alvarenga and Saes, 1992; Alvarenga et al.,
2000). In general, the basement of the Corumba
Group/Puga Formation comprises igneous and
metamorphic rocks older than the Brasiliano-Pan
African orogeny, recording a widespread tecto-
nothermal event in the Mesoproterozoic (ca. 1.3�/
1.0 Ga).
The Corumba Group was deposited on a stable
continental margin (Almeida, 1984: 274; Zaine,
1991; Boggiani et al., 1993, 1998; Boggiani, 1998),
and includes alternating siliciclastic and carbonatic
units with a composite thickness of approximately
600 m (Boggiani, 1998). Following glacial and
glaciomarine deposits of the Varangerian/Mar-inoan Puga Formation (Almeida, 1984; Alvar-
enga, 1990; Alvarenga and Trompette, 1992;
Alvarenga et al., 2000), a siliciclastic deepening-
upward sequence was deposited, represented by
the Cerradinho Formation (Almeida, 1965, 1984;
Boggiani, 1998). Boggiani (1998) interpreted the
Cadiueus Formation as alluvial fan, synrift depos-
its generated during opening of the Corumbabasin. While this explanation cannot be ruled
out, it must be born in mind that the underlying
Puga Formation already records (glacio) marine
conditions in the shelf. Furthermore, it is well
known that alluvial fan deposits are very similar to
glacial-outwash fans, also termed sandur deposits
(Ruegg, 1977; Reineck and Singh, 1980: 202;
Miall, 2000). Therefore, the Cadiueus Formationprobably represents glacial-outwash fans depos-
ited during retreat of the Varangerian ice-sheet.
The Cerradinho Formation conformably passes
into carbonates of the Bocaina Formation, or
more rarely, directly into the Tamengo Formation
(Almeida, 1965, 1984; Boggiani, 1998; Fig. 4). The
Bocaina Formation is characterized by thick
deposits of stromatolitic dolostones, that show agreat lateral variation of facies (Boggiani et al.,
1993; Boggiani, 1998), including phosphorites with
up to 34% P2O5. The overlying Tamengo Forma-
tion is mainly composed of dark, organic rich
limestones and marls with occasional Cloudina -
event accumulations (Almeida, 1965, 1984; Zaine,
1991; Boggiani, 1998). Dark limestones and lime-
stone�/dolostone rhythmites included by Almeida(1984) in the Cerradinho Formation were consid-
ered by Gaucher (2000: 107) as part of the basal
Tamengo Formation. Finally, carbonates of the
Tamengo Formation are concordantly overlain by
gray siltstones of the Guaicurus Formation (Al-
meida, 1965; Boggiani, 1998), marking the end of
deposition in the Corumba basin.
The Corumba Group was deformed and meta-morphosed in the Cambrian-lowermost Ordovi-
cian (540�/490 Ma) during the Paraguay Orogeny,
as defined by Basei and Brito Neves (1992). This
orogenic event has been either considered as a
terminal event of the Brasiliano-Pan African
Megacycle (Almeida, 1984; Alvarenga and Tromp-
ette, 1993; Trompette in Alvarenga et al., 2000) or
as a separate orogeny (Basei and Brito Neves,1992: 333; Gaucher, 2000; see above). Almeida
(1984) defined two main structural domains in the
Paraguay Belt, based on observed tectonic style,
metamorphism and lithology. The ‘Metamorphic
Brazilides’ represent the most deformed and me-
tamorphosed domain to the E, in contrast with the
‘Non-metamorphic Brazilides’, which include the
less deformed regions adjacent to the craton(Almeida, 1984; Fig. 3). This scheme proved very
successful in explaining the different tectonic styles
that shows the Corumba Group in the different
domains, and has been adopted by most authors
dealing with the Paraguay Belt (Almeida, 1984;
Boggiani et al., 1993; Alvarenga and Trompette,
1993; Boggiani, 1998; Alvarenga et al., 2000).
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 245
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Granitogenesis in the Paraguay Belt is mainlyconfined to the ‘Metamorphic Brazilides’. Post-
tectonic granites such as the Sao Vicente Granite
yielded ages of 5049/12 Ma (K/Ar in biotite:
Almeida and Mantovani, in Boggiani, 1998: 24),
giving minimum age-constraints for the Corumba
Group.
3. Lithostratigraphy and sedimentary environments
3.1. Comparative lithostratigraphy
3.1.1. Common features
The comparison of both sedimentary sequences
reveals the following common features of the
Arroyo del Soldado and Corumba groups:
(1) Both successions rest with erosional andangular unconformity on an Archean to Meso-
proterozoic granitic�/metamorphic basement that
is consistently older than the Brasiliano-Pan
African Orogeny (:/750�/600 Ma: Campos Neto
(2000)), a fact of great importance for palaeogeo-
graphic reconstructions of the region.
(2) Both groups begin with a siliciclastic, dee-
pening-upward sequence unconformably overlyingpre-Brasiliano basement, represented by the Yer-
bal and Cerradinho formations. These units con-
tain rather thin, mature, basal conglomerates and
sandstones, with complete absence of volcanics,
volcanoclastics and pyroclastites (Almeida, 1984;
Boggiani et al., 1993; Boggiani, 1998; Gaucher et
al., 1998a,b; Gaucher, 1999, 2000). These facts
indicate that the basin was tectonically stable andhad a low palaeorelief, suggesting that the cause of
the transgression was an eustatic sea-level rise
rather than extensional tectonics. At the top, the
mentioned formations are composed mainly of
green, chloritic and glauconitic siltstones, which
contain an acritarch-assemblage strongly domi-
nated by Bavlinella faveolata (Gaucher, 1999,
2000).(3) The siliciclastic transgressive units of both
groups pass to the top with sharp contact into
thick carbonate deposits (Polanco and Bocaina-
Tamengo Formations; Fig. 4). The Polanco and
Tamengo formations are mainly made up of dark,
organic-rich, clastic limestones and limestone�/
dolostone rhythmites (Boggiani, 1998; Gaucher,
1999, 2000). The carbonatic rhythmites are very
distinctive lithologies, that have been cited only
from few places around the world (Bose, 1979;
Fairchild, 1980; Gaucher, 1999, 2000), and indi-
cates that both the Arroyo del Soldado and
Corumba groups presented the special conditions
necessary for their genesis. Sedimentary structures
and facies indicate that the Tamengo Formation
was deposited in a shallower basin than the
Polanco Formation. Tempestites are frequent in
both units, as documented by occurrence of
graded limestone beds with hummocky cross-
stratification and event-accumulations of Cloudina
in the Corumba Group (Zaine, 1991; Boggiani et
al., 1993; Boggiani, 1998; Gaucher, 2000). These
facts indicate the common occurrence of storms in
both basins, probably due to a tropical setting
(Seilacher and Aigner, 1991).
(4) Carbonate deposits of the Tamengo and
Polanco Formations are overlain by thick silici-
clastic, mainly pelitic units, namely the Guaicurus
and Cerro Espuelitas Formations, respectively
(Boggiani, 1998; Gaucher, 2000). These units are
made up mainly of siltstones and shales, with
minor carbonates at the base. Thick chert and BIF
deposits of the middle Cerro Espuelitas Formation
are absent in the Guaicurus Formation. A possible
explanation could be that only the lower portion
of the Cerro Espuelitas Formation is represented
in the Corumba Group, as also suggested by the
fossils that occur in the Guaicurus Formation. The
chemostratigraphic data obtained so far support
these ideas as well (see below).(5) Palaeocurrents measured in the Corumba
Group consistently indicate that the basin dee-
pened to the east (Boggiani, 1990, 1998; Alvar-
enga, 1990; Boggiani et al., 1993) and point to a
source-area in the Rıo Apa Block, to the west
(Figs. 1 and 3). In the case of the ASG, palaeo-
currents measured also indicate a shelf that
deepened to the SE�/E, with a palaeoshoreline
roughly N�/S and source areas to the west
(Montana and Sprechmann, 1993; Gaucher,
2000). Facies arrangement and thickness in both
basins corroborate palaeocurrent data.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278246
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3.1.2. Differences
There are some important differences in the
lithostratigraphy of the Corumba and ASGs which
need to be considered here:
(1) A clear record of glacial episodes in SW-
Brazil is provided by the Jacadigo Group and by
tillites of the Puga Formation (Almeida, 1984;
Alvarenga, 1990; Alvarenga and Trompette, 1992;Alvarenga et al., 2000). Evidence of glaciation is so
far absent in the ASG. The Puga Formation is
composed of massive, polimictic diamictites in-
cluding angular, faceted and striated clasts up to
some decimeters in size (Almeida, 1984: 272�/
273Alvarenga, 1990; Zaine, 1991; Alvarenga and
Trompette, 1992; Boggiani, 1998). Intercalations
of pelites and fine sandstones including decimetricdropstones also occur (Almeida, 1984; Fig. 4) and
corroborate the interpretation of a glacial, most
probably glaciomarine depositional environment
for the unit (Alvarenga, 1990; Alvarenga and
Trompette, 1992; Alvarenga et al., 2000). Taking
into account that the Puga Formation is concor-
dantly overlain by the upper Vendian Corumba
Group (Almeida, 1984; Boggiani, 1998; Alvarengaet al., 2000), these deposits represent the record of
the Varangerian/Marinoan Ice Age (Hambrey and
Harland, 1985; Harland, 1989; Kasting, 1992;
Kaufman et al., 1997; Saylor et al., 1998; Knoll,
2000) in the Corumba basin (Alvarenga, 1990: 34;
Alvarenga and Trompette, 1992). As noted above,
the Cadiueus Formation may represent glacial-
outwash deposits generated during the retreat ofthe ice-sheet. Two main scenarios can be proposed
to explain the absence of Varangerian or older
glacial deposits in the ASG, namely: (a) a tropical
palaeogeographic location, inside the circum-
equatorial ice-free zone (Hyde et al., 2000; Run-
negar, 2000), where glaciation only led to sea-level
drop, leaving no glacial deposits; and (b) glacia-
tion with complete erosion of the sedimentaryrecord of the Varangerian in the basin. Of these
scenarios, the former is prefered because there is
indeed abundant evidence of a tropical setting for
the ASG, but complete erosion of the record
cannot be ruled out (Montana and Sprechmann,
1993; Gaucher et al., 1996; Gaucher, 1999, 2000).
(2) Sedimentary structures indicate that the
Corumba Group was deposited in a shallowerbasin, leading to a greater facies-variation com-
pared to the ASG. While the Tamengo Formation
shows a quite rich facies variability (Boggiani,
1998; Boggiani et al., 1993), the Polanco Forma-
tion presents a great lateral facies-persistence
(Gaucher, 1999, 2000). Composite thickness of
the ASG exceeds 5000 m (Gaucher, 2000) and is
Fig. 3. Generalized geological map of SW-Mato Grosso do
Sul, showing broad distribution of lithologies of the Corumba
Group, adapted from Boggiani (1998). Occurrences of other
Neoproterozoic-Cambrian units is also shown. Numbers in-
dicate sites referred to in the text.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 247
Page 8
thus roughly 10 times greater than that of the
Corumba Group (600 m: Boggiani, 1998), in
accordance with deposition in a deeper and more
rapidly subsiding basin.(3) The stromatolitic and oolitic dolostones of
the Bocaina Formation (sensu Boggiani et al.,
1993; Boggiani, 1998) do not have a counterpart in
the ASG. Absence of stromatolites at the base of
the Polanco Formation is due to a deeper sedi-
mentary environment. Oolitic calcarenites have
been found, however, in the Polanco Formation
in the Isla Cristalina de Rivera (Gaucher, 2000;
Fig. 2). Moreover, the fact that the Cerradinho
Formation passes directly into non-stromatolitic
carbonates of the Tamengo Formation at many
localities indicates that the Bocaina Formation
could be restricted to the shallowest areas of the
Corumba basin. Boggiani (1998: 64) proposed that
the Bocaina Formation represents the transgres-
sion of the Vendian Sea onto a peneplainized
basement (Pedra Branca surface) generating a
large and shallow epicontinental basin. In deeper
sections like in the Serra da Bodoquena, carbo-
nates of the Tamengo Formation rest directly on
the Cerradinho Formation, closely parallelling the
lithostratigraphy of the ASG (Gaucher, 2000). At
Fazenda Baıa das Garcas (points 3�/4, Fig. 3), for
instance, the upper Cerradinho Formation passes
directly into dark limestones and limestone�/do-
lostone rhythmites, stromatolitic dolostones of the
Bocaina Formation being completely absent at
that locality (Boggiani, 1990, 1998; Zaine, 1991;
Gaucher, 1999, 2000).
(4) Black phosphorites with up to 34% P2O5
have been reported from the Bocaina and basal
Tamengo Formations (Boggiani et al., 1993;
Boggiani et al., 1998). They are made up of
cryptocrystalline fluorapatite and contain large
amounts of organic matter (�/5%). Such litholo-
gies have not been found so far in the ASG, where
the only traces of phosphate are relicts in the shells
of Waltheria marburgensis at the top of the Yerbal
Formation (Gaucher and Sprechmann, 1999;
Gaucher, 2000). As known from recent examples,
phosphorite deposits are mainly the consequence
of upwelling of deep water at the western margins
of continents under favourable wind and current
regime (Baturin, 1982). Evidence of upwelling in
the upper Yerbal Formation has been presented byGaucher (1999, 2000), including acritarch assem-
blages indicative of basin eutrophication, occur-
rence of glauconitic and chamositic siltstones,
banded ferruginous siltstones, and recently dis-
covered BIF. Considering the phosphorite occur-
rences in the Corumba Group restricted to rather
thin beds (Fig. 4) and the mentioned evidences of
upwelling in the ASG, we conclude that the
occurrence of phosphatic rocks in the latter unit
cannot be ruled out. Furthermore, upwelling
played indeed a major role in both groups at the
contact between basal siliciclastics and overlying
carbonates, probably indicating a similar palaeo-
geography for both basins.
(5) The ASG includes thick deposits of chert and
BIF in the Cerro Espuelitas Formation (Gaucher
and Schipilov, 1994; Gaucher et al., 1996, 1998b;
Gaucher, 1999, 2000), which are unknown in the
Corumba Group (Fig. 4). The BIF represent
micro- and meso-banded, oxide-facies BIF
(Beukes, 1973; James, 1983) containing up to 35
wt.% magnetite or hematite and occurring in up to
three horizons with maximum thickness of 200 m.
Thickness of banded, massive and ferruginous
cherts of the middle Cerro Espuelitas Formation
reaches 400 m (Gaucher et al., 1996, 1998b;
Gaucher, 1999, 2000). The Guaicurus Formation
is probably correlative to the lowermost Cerro
Espuelitas Formation, where BIF and chert are
absent (Gaucher 1999, 2000). As already men-
tioned above, BIF of the Jacadigo Group cropping
out in the region of Corumba, are considered older
than both the Corumba and ASG, being probably
of Varangerian or even Sturtian age (Almeida,
1984; Walde, 1987; Zaine, 1991; Gaucher, 2000).
Fig. 4. Composite stratigraphic columns of the Corumba and ASGs, showing correlations between lithostratigraphic units and
stratigraphic distribution of most important fossil-taxa (sources: Boggiani, 1998; Gaucher, 2000 and this work). CV, Cerro Victoria
Formation; Cerros SF, Cerros San Francisco Formation; Cad, Cadiueus Formation. The Cadiueus Formation has not been drawn to
avoid a more complicated representation (see text). Shaded intervals in the ASG represent periods of basin eutrophication.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278248
Page 9
Fig. 4
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 249
Page 10
(6) Gaucher (2000) recognized an impressiveclimatic deterioration recorded at the boundary
between the Polanco and Barriga Negra forma-
tions, on the basis of: (a) termination of carbonate
deposition at the top of the Polanco Formation;
(b) change of clay-mineral composition of silt-
stones; and (c) destabilization of the shallower
areas of the Arroyo del Soldado shelf due to sea-
level drop, leading to cannibalization of shelfdeposits and local deposition of up to 1500 m of
conglomerates of the Barriga Negra Formation.
These conglomerates do not have a correlative in
the Corumba Group, although the basin was
shallower than the Arroyo del Soldado basin.
Possible explanations could be that: (1) palaeor-
elief was much steeper in some parts of the Arroyo
del Soldado basin than in the Corumba basin; (2)the regression left only an erosive record in the
Corumba Group; and/or (3) influence of isostatic
reactivations of the Sarandı del Yı-Piriapolis
lineament were also responsible for the genesis of
the conglomerates, a factor absent in the Corumba
basin.
3.2. Geotectonic setting and sedimentary
environments
The analysis of the preceding data indicates that
the Corumba and ASGs were deposited in similar
sedimentary environments and geotectonic set-
tings. Both units were deposited on a stable,
Atlantic-type continental margin, in a predomi-
nantly marine environment (Almeida, 1984: 274;
Zaine, 1991; Boggiani et al., 1993; Gaucher et al.,1996, 1998b; Boggiani, 1998; Gaucher, 1999,
2000). Evidence of tectonic stability in both basins
is provided by: (a) the absence of volcanic,
pyroclastic or volcanoclastic rocks; (b) textural
characteristics of sandstones, which are mainly
quartz-arenites and subarkoses (Pettijohn et al.,
1987); (c) provenance of sandstones and conglom-
erates from a deeply eroded, granitic and(para)metamorphic source-area; and (d) develop-
ment of extensive carbonate ramps and stromato-
lite buildups.
The Corumba and ASGs were deposited in a
marine sedimentary environment, as shown by
microfossil assemblages, skeletal fossils, vendotae-
nids (see below), sedimentary structures, petrogra-phy and facies distribution. Both basins record the
transgression that followed the termination of the
Varangerian glaciation, as indicated by biostrati-
graphy and carbon isotopes. This is specially clear
in the Corumba Group, where this glacial episode
is represented by the tillites of the underlying Puga
formation (Almeida, 1984; Alvarenga, 1990). The
glacioeustatic sea-level rise that followed theVarangerian determined the deposition of the
basal units of both groups. In fact, the major
control of sedimentation was provided by (gla-
cio)eustasy and not by tectonic processes, in
accordance with the proposed geotectonic setting.
Evidences of drastic climatic changes coupled with
eustatic sea-level oscillations are widespread in the
Corumba and ASGs. The lower portions of thebasal Cerradinho and Yerbal formations were
deposited under cool and/or arid conditions, as
shown by sandstone composition (Boggiani, 1998:
44; Gaucher, 1999, 2000) and clay-mineral suites
present (Gaucher, 1999, 2000). At the base of the
Bocaina, Tamengo and Polanco Formations, cli-
mate shifted towards warm, tropical conditions,
because: (a) oolites are widespread (Zaine, 1991;Boggiani, 1998; Gaucher, 2000), which are typical
of tropical regions (Fuchtbauer and Richter, 1988:
335); (b) abundant carbonatic tempestites in the
Tamengo and Polanco Formations (Boggiani et
al., 1993; Boggiani, 1998; Gaucher, 2000) indicate
recurrent large storms which are more common at
low latitudes (Seilacher and Aigner, 1991: 252); (c)
clay-mineral and terrigenous are composed mainlyof kaolinite (Gaucher, 1999, 2000) and quartz-
arenite, respectively (Boggiani, 1998: 68�/69; Gau-
cher, 1999, 2000), suggesting intense chemical
weathering in the source-areas (Pettijohn et al.,
1987; Heling, 1988); and finally (d) the huge
amounts of carbonate deposited in both basins
are also indicative of a tropical setting. Up section,
climate deteriorated again, as shown by the inter-ruption of carbonate deposition at the transition
to the Guaicurus and Barriga Negra�/Cerro
Espuelitas formations. Gaucher (1999, 2000)
showed that this climate change caused an im-
portant regression which exposed the shallower
areas of the Arroyo del Soldado shelf. An asso-
ciated negative d13C-excursion in unaltered carbo-
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278250
Page 11
nates (see below), also suggests that this regressioncould be the consequence of global cooling or even
glaciation (Kaufman et al., 1997; Jacobsen and
Kaufman, 1999; Knoll, 2000; Kaufman, 2000).
Upwelling was another important factor in both
shelves, leading to deposition of BIF, chert and
phosphorites, causing phytoplankton blooms and
eutrophication of the basins (Boggiani et al., 1993;
Gaucher et al., 1996: 363; Boggiani, 1998; Gau-cher, 2000; Fig. 4). This fact indicates that the
palaeogeographic setting of both basins was very
similar, with a combination of shoreline orienta-
tion, current and wind regime that promoted
upwelling of deep, cold seawater (Baturin, 1982).
Indications that the basin was stratified and had
episodic periods of eutrophication related to
upwelling are: (a) BIF occurring in the ASG andphosphorites in the Corumba Group; (b) strong
dominance of the low-diversity microfossil assem-
blages by one or two taxa, including Bavlinella
faveolata (Gaucher, 2000; Fig. 4), as also found for
other upper Vendian successions worldwide (Man-
suy and Vidal, 1983; Palacios, 1989; Vidal and
Nystuen, 1990; Steiner, 1994); and (c) occurrence
of organic-rich horizons with up to 11% TOCsuggesting high bioproductivity and enhanced
preservation of organic matter.
A substantial difference observed is, that while
the ASG was deposited in a typical marginal or
pericontinental sea (sensu Einsele, 1992), the
Corumba Group was in part deposited in a
shallower epicontinental sea over a peneplainized
craton. This could explain the greater thickness ofthe ASG, as well as its more complete sedimentary
record (Einsele, 1992: 124). The Arroyo del
Soldado basin was steeper and responds to a
typical marginal sea (Einsele, 1992), with great
lateral facies-persistence over hundreds of kilo-
meters and deposition of a thick sedimentary
wedge.
4. Palaeontology
4.1. Previous work
The first fossils to be reported from the units
considered here were calcareous tubes of the
Tamengo Formation described by Beurlen andSommer (1957) as Aulophycus lucianoi , and later
included in the genus Cloudina by Zaine and
Fairchild (1985) and Hahn and Pflug (1985).
Hahn et al. (1982) reported the occurrence of
possible Scyphozoa in the Tamengo Formation,
erecting the taxon Corumbella werneri . The occur-
rence of acritarchs in the Corumba Group was
first reported by Fairchild and Sundaram (1981)and later confirmed by Zaine and Fairchild (1987),
which assigned them to the species Bavlinella
faveolata and cf. Vandalosphaeridium sp. The first
systematic study of the palaeontology of the
Corumba Group was made by Zaine (1991), who
described acritarchs, filamentous microfossils, ven-
dotaenids, and possible metazoans of the genus
Cloudina and Corumbella . Boggiani (1998) re-ported new occurrences of Cloudina and Corum-
bella , extending the known stratigraphic range of
these taxa in the Corumba Group. Gaucher (2000)
describes Eoholynia sp. from the lower Guaicurus
Formation. Finally, Fairchild et al. (2000) men-
tioned the occurrence of possible ichnofossils,
newly discovered stromatolites, oncolites and
silicified colonies of probably cyanobacterial affi-nities.
Palaeontologic studies began more recently in
the ASG. Montana and Sprechmann (1993) re-
ported the occurrence of stromatolites and ichno-
fossils in the Cerro Victoria Formation. Gaucher
and Schipilov (1994) described the first acritarchs
from BIF of the Cerro Espuelitas Formation. A
number of publications by Gaucher et al. (1996,1998b) and Gaucher and Sprechmann (1998)
followed, describing organic-walled microfossils
from almost the entire ASG. An assemblage of
five genera and species of skeletal fossils including
Cloudina riemkeae Germs (1972) has been re-
ported by Gaucher and Sprechmann (1999). Fi-
nally, Gaucher (2000) presented detailed
palaeontologic and biostratigraphic informationfor the ASG, also describing fossil material from
the Corumba Group.
In the following section, new fossil material
from the Corumba Group is described and com-
pared with the fossil assemblages of the ASG. All
specimens described are deposited in the collection
of the Departamento de Paleontologıa of the
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 251
Page 12
Facultad de Ciencias (FCDP) in Montevideo(Uruguay).
4.2. Organic-walled microfossils
Organic-walled microfossils are abundant and
well preserved in the ASG. Although microfossils
and kerogens are also abundant in the Corumba
Group, preservation is quite poor due to carboni-
zation attributed to a thermal event associatedwith generation of anorogenic basaltic dykes
(Walde, 1987) or unspecified contact metamorph-
ism (Trompette et al., 1998: 595). This thermal
event also affected the Mn-ore paragenesis of the
Jacadigo Group, as well as their oxygen isotopic
composition (Trompette et al., 1998 and references
therein). Therefore, the reported taxa should be
taken only as a fragment of the original micro-biota, because most of the organic remains are
unidentifiable.
Kingdom Eubacteria Woese & Fox, 1977
Phyllum Cyanobacteria Stanier et al., 1978
Classis, Ordo et Fam. indet.
Genus Bavlinella (Schepeleva) Vidal, 1976
Type species: Bavlinella faveolata (Schepeleva)
Vidal, 1976
Bavlinella faveolata (Schepeleva), Vidal, 1976
Fig. 5C�/E, G�/H; Fig. 6F
1974 Sphaerocongregus variabilis Moorman: pls. 1�/3
1976 Bavlinella faveolata Vidal: figs. 7A�/C
1990 Sphaerocongregus variabilis Vidal & Nystuen:
fig. 9A�/B, D�/E, G�/L
1991 Sphaerocongregus variabilis Zaine: fig. 2A.
1992 Bavlinella faveolata Schopf: pl. 54J1�/J3
1996 Bavlinella faveolata Gaucher et al.:
figs. 7.1�/7.2
2000 Bavlinella faveolata Gaucher: pl. 9, pl. 18.1�/18.2
Type specimen. Vidal (1976) adopted the diag-
nosis given by Moorman (1974) for Sphaerocon-
gregus variabilis as the valid diagnosis for
Bavlinella faveolata . Vidal and Nystuen (1990)
find that the type specimen illustrated by Shepe-
leva (1962), in Vidal, 1976) ‘is in fact the organic
residue after maceration of framboidal pyrite’, andrecommend the use of the junior synonim instead
of Bavlinella faveolata for this species. Never-
theless, German et al. (1989) had already desig-
nated a lectotype for the species from the Kotlin
Formation of the former USSR. This lectotype has
been also illustrated by Schopf (1992: pl. 54-J).
Therefore, the valid designation of a lectotype
supersedes any previous restriction of the applica-tion of the name of the genus and species
Bavlinella faveolata . Sphaerocongregus variabilis
Moorman (1974) is thus to be considered as a
junior synonim.
Material. Hundreds of specimens in thin-sec-
tions of siltstones and palynological macerations
of limestones and marls. In the siltstones, the
specimens are commonly permineralized withiron-oxides (Fig. 5G�/H). The species often occurs
in acid macerations, showing advanced carboniza-
tion which gives the vesicles a gray to black colour
(Figs. 5D and 6F).
Description. The observed specimens are single
spheroidal vescicles made up of hundreds of
tightly packed, micron-sized microspheres, thus
corresponding to the endosporangia morphotypeof Moorman (1974).
Dimensions. Specimens preserved in green silt-
stones of the upper Cerradinho Formation range
in diameter between 4.0 and 21.5 mm (mean�/9.9
mm, S.D.�/4.0 mm, N�/40). This population was
measured mainly in thin sections, but specimens in
palynological macerations were also considered.
On the other hand, the population palynologicallyrecovered from marls of the Tamengo Formation
show larger diameters, between 16 and 45 mm
(mean�/27.9 mm, S.D.�/7.8 mm, N�/13). Zaine
(1991: 109) found vesicles with diameters ranging
between 4 and 15 mm (mean�/7 mm, N�/20) in
macerations of the same stratigraphic levels of the
Tamengo Formation. This strongly suggests that
the differences in mean size observed are due eitherto facies dependence of the populations or to
preparation biases (i.e. mesh apertures of sieves
used).
Distribution. The occurrence of this taxon in the
Tamengo Formation was already noted by Zaine
and Fairchild (1987) and Zaine (1991). Bavlinella
faveolata occurs in large masses in green siltstones
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278252
Page 13
of the upper Cerradinho Formation in the Serra da
Bodoquena (point 3, Fig. 3), and also in black,
organic-rich marls of the Tamengo Formation
near Corumba (Pedreira Saladeiro/Itau, point 9
of Fig. 3), as described by the above mentioned
authors.
Remarks. Bavlinella faveolata shows more
variability and better preservation in the ASG,
with occurrence of all morphotypes described by
Moorman (1974), as noted by Gaucher (2000).
Apart from this, the fossils of the Corumba Group
are identical to the corresponding morphotypes of
the ASG. Bavlinella faveolata is closely associated
with green siltstones in the Cerradinho Formation
occurring in large numbers and dominating the
fossil assemblage. In this facies, Bavlinella is
Fig. 5. Organic-walled microfossils from the Corumba and ASGs, as observed in thin sections (C, E�/H) and palynological
macerations (A�/B, D). (A) Vendotaenia antiqua , fragment from green siltstones of the upper Cerradinho Formation (point 3, Fig. 3).
(B) Saucer-shaped colony of Soldadophycus bossii from gray siltstones of the lowermost Guaicurus Formation (Pedreira Laginha,
point 10 of Fig. 3). Note typical transitions between spheroids and filaments. (C) Bavlinella faveolata , vesicle-chain occurring in green
siltstones of the upper Cerradinho Formation (point 3, Fig. 3). (D�/E) Solitary vesicles of Bavlinella faveolata from the same facies and
locality as C. (F) Bavlinella faveolata , specimen FCDP 3189 from the Yerbal Formation of the ASG (near Minas de Corrales). (G�/H)
Solitary, hematized vesicles of Bavlinella faveolata , occurring in green siltstones of the upper Cerradinho Formation (point 3, Fig. 3).
Note loss of details due to ferrification. Scale bars represent 10 mm for all figures.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 253
Page 14
Fig. 6. Organic-walled microfossils recovered by means of palynological macerations from the Corumba Group. (A) Soldadophycus
bossii , carbonized, saucer-shaped colony from gray siltstones of the lowermost Guaicurus Formation at point 10 (Fig. 3). (B)
Soldadophycus bossii , same procedence as the former. (C�/D) Two filament-balls of Siphonophycus robustum from gray siltstones of the
lowermost Guaicurus Formation at point 10 (Fig. 3). (E) Myxococcoides sp., semicarbonized colony in dark marls of the Tamengo
Formation (Pedreira Saladeiro/Itau, point 9 of Fig. 3). Note psilate, loosely aggregated spheroids (arrowed). (F) Bavlinella faveolata ,
single vesicle from the Tamengo Formation (same procedence as E). (G) Leiosphaeridia sp., carbonized specimen under epi-
illumination. Recovered from gray siltstones of the lowermost Guaicurus Formation (point 10, Fig. 3). Scale bars represent 10 mm for
all specimens.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278254
Page 15
typically hematized, sometimes obscuring mor-phologic detail (Fig. 5G�/H). The same happens
in the correlative Yerbal Formation and also in the
Cerro Espuelitas Formation of the ASG (Gaucher,
2000). The palynofacies are in fact almost indis-
tinguishable, probably due to eutrophic conditions
fueled by high nutrient (P, Fe, N) availability in
both basins.
Bavlinella faveolata is a long-ranging fossil,
found in successions ranging from the UpperRiphean (Vidal, 1976; Samuelsson, 1997) to the
Ordovician (Reitz, 1991). Well-preserved fossils
clearly belonging to this taxon have been found by
Salamon and Gaucher (in preparation) in Middle
Devonian (Givetian) siliceous shales of the Rhei-
nisches Schiefergebirge of Germany. Nevertheless,
it is clear that the taxon reached its acme in the
upper Vendian, when it was a dominating compo-nent of the biota worldwide (Moorman, 1974;
Mansuy and Vidal, 1983; Knoll and Sweet, 1985;
Germs et al., 1986; Palacios, 1989; Vidal and
Nystuen, 1990; Gaucher, 2000).
Genus Siphonophycus Schopf (1968), emend Knoll
et al. (1991)
Type species: Siphonophycus kestron Schopf, 1968
Siphonophycus robustum Schopf (1968), emend
Knoll et al. (1991)
Fig. 6C�/D
1968 Eomycetopsis robusta Schopf: pls. 82.2�/3; 83.1�/4
1991 Siphonophycus robustum Knoll et al.: figs. 10.3, 10.5
1994 Siphonophycus robustum Butterfield et al.: figs. 26A, G
1994 Siphonophycus robustum Hofmann and Jackson: fig. 11.5
Type specimen: Specimen of plate 83.1 (a, b) of
Schopf (1968)
Material. Three mat fragments and one com-
plete filament ball in macerations of gray siltstones
of the base of the Guaicurus Formation atPedreira Laginha, near Corumba (point 10, Fig.
3).
Description. Psilate, unbranched, nonseptate
filaments 0.8�/3.0 mm in diameter (mean�/1.8
mm, S.D.�/0.5 mm, N�/21). The entangled fila-
ments occur either as irregular mat-fragments or
as filament balls (Fig. 6C�/D).
Remarks. The species of the genus Siphonophy-
cus are distinguished on the basis of trichome-size
(Knoll et al., 1991; Butterfield et al., 1994). The
material from the Guaicurus Formation has a size
distribution intermediate between S. septatum
(Schopf) Knoll et al. (1991) and S. robustum .
The latter is characterized by filament diameters in
the range of 2�/4 mm (Knoll et al., 1991; Butterfield
et al., 1994; Hofmann and Jackson, 1994). Never-theless, the habit of the filament-aggregates de-
scribed here is identical to S. robustum . Butterfield
et al. (1994: fig. 26G) report the occurrence of S.
robustum -balls similar to modern Nostoc-balls in
the Svanbergfjellet Formation of Spitsbergen,
which closely resemble our material. Moreover,
they find that the species is the most common
shale-facies microbial mat-builder, as is the case inthe Guaicurus Formation. Therefore, we assign
the material to S. robustum (Schopf) Knoll et al.
(1991).
Incertae sedis
Group Acritarcha Evitt, 1963
Genus Leiosphaeridia Eisenack (1958), emend
Downie & Sarjeant (1963)
Type species: Leiosphaeridia baltica Eisenack
(1958)
Leiosphaeridia tenuissima Eisenack (1958)
1958 Leiosphaeridia tenuissima Eisenack: pl. 1.2�/1.3
1994 Leiosphaeridia tenuissima Butterfield et al.: fig. 16I
1994 Leiosphaeridia tenuissima Hofmann and Jackson: fig.
12E
1998 Leiosphaeridia tenuissima Gaucher et al.: fig. 4.6
2000 Leiosphaeridia tenuissima Gaucher: pl. 11.5
Material. Two well-preserved specimens in ma-
cerations of siltstones of the lowermost Guaicurus
Formation at Pedreira Laginha (point 10, Fig. 3).
Description. Thin-walled, compressed, psilate
spheroidal vesicles with common folds. Diameter
ranging from 70 to 120 mm (only two specimens).The individuals are partially carbonized, showing
a light gray colour.
Remarks. These are the largest acritarchs of the
Corumba microbiota, as is also the case in the
ASG (Gaucher, 2000). Absence of larger acri-
tarchs is typical of uppermost Vendian successions
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Page 16
(Knoll, 1996b; Vidal and Moczydlowska-Vidal,1997).
Leiosphaeridia sp.Fig. 6G
2000 Leiosphaeridia sp. Gaucher: p. 105.
Material. Two complete specimens and some
fragments recovered by acid maceration from
siltstones of the lowermost Guaicurus Formation
at Pedreira Laginha (point 10, Fig. 3).Description. Compressed ellipsoidal vesicles
with rectilinear or curvilinear folds, 72�/150 mm
in maximum diameter. Wall probably thick, and
opaque due to advanced carbonization. Irregular
sculpture of wall probably due to degradation of
an originally psilate wall.
Remarks. Due to the opacity of the vesicles, no
reliable estimation of wall-thickness could bemade. Nevertheless, the specimens are quite dif-
ferent in their degree of carbonization, type of
folds and superficial features to those described
here under Leiosphaeridia tenuissima from the
same locality. These facts suggest that the species
is thicked-walled, unlike L. minutissima . If this
point is confirmed, then the specimens could be
assigned to Leiosphaeridia jacutica (Timofeev)Mikhaylova and Yankauskas (Hofmann and
Jackson, 1994), but more material is needed.
Incertae sedis
Genus Soldadophycus Gaucher et al. (1996)
Type species: Soldadophycus bossii Gaucher et al.
(1996)
Soldadophycus bossii Gaucher et al. (1996)Fig. 5B, Fig. 6A�/B
1989 Tipo B Palacios, pl. V; figs. 1�/4
1996 Soldadophycus bossii Gaucher et al.: figs. 6.1�/6.5; fig.
6.7
1998b Soldadophycus bossii Gaucher et al.: Figs. 4.7�/4.8
1998 Soldadophycus bossii Gaucher and Sprechmann: p. 184
2000 Soldadophycus bossii Gaucher: pls. 14�/15; 17.4
Type specimen. Holotype: specimen FCDP3188, figured by Gaucher et al. (1996: fig. 6.1).
Paratype of saucer-shaped colonies is the specimen
FCDP 3207b (fig. 6.2 of Gaucher et al., 1996).
Material . Twelve colonies and colony-fragments
in palynological macerations of marls, siltstones
and limestones of the upper Cerradinho, Tamengo
and lower Guaicurus formations. Fossil sites for
each of the mentioned units are the locality of Baıadas Garcas (point 3, Fig. 3), Pedreira Saladeiro/
Itau (point 9, Fig. 3) and Pedreira Laginha (point
10, Fig. 3), respectively. The species occurs more
frequently in the Cerradinho and Guaicurus for-
mations.
Dimensions . Diameter of the spheroidal cells
ranging between 2.6 and 8.2 mm (mean�/4.9 mm,
S.D.�/1.6 mm, N�/41). Maximum width of fila-ments varying between 1.7 and 3.5 mm (mean�/2.3
mm; S.D.�/0.6 mm; N�/7). Diameter of saucer-
shaped colonies varying between 57 and 65 mm. A
single vase or bottle-shaped colony occurs, with a
diameter of 90 and 160 mm long. These values are
completely within the cell and colony sizes typical
for the species (Gaucher et al., 1996; Gaucher,
2000: 78).Description . Soldadophycus bossii is character-
ized by the co-occurrence of psilate, spheroidal
cells and septate, branched filaments (Gaucher et
al., 1996). Nevertheless, some colony-types are
made up only of spheroidal cells. In the Corumba
Group, the saucer-shaped colonies dominate (Figs.
5B and 6A�/B), but subspheroidal and vase-shaped
colonies also occur. The typical transitions fromspheroids into filaments and vice versa were also
observed (Fig. 5B). Unlike in the ASG, the fossils
are always carbonized, showing gray to black
(opaque) colours.
Remarks. As in the ASG, Soldadophycus occurs
in the Corumba Group in relatively large numbers
only in the siliciclastic units bracketing carbonate
deposits (Cerradinho and Guaicurus formations).In the Tamengo Formation they are an accessory
element of the microbiota, as in the case of the
Polanco Formation (Gaucher, 2000).
Genus Myxococcoides Schopf (1968)
Type species: Myxococcoides minor Schopf (1968)
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Page 17
Myxococcoides sp.
Fig. 6.E
2000 Myxococcoides sp. Gaucher: p. 105
Material . Three fairly well preserved colonies in
palynological macerations of marls of the Ta-mengo Formation (Pedreira Saladeiro/Itau, point
9 of Fig. 3) and green siltstones of the Cerradinho
Formation (point 3, Fig. 3).
Description. Spheroidal to ellipsoidal, colonial
microfossils. Walls thin, psilate, and rather hyaline
despite advanced carbonization. Cells loosely
aggregated into irregular colonies of up to 50 mm
in maximum dimension. Maximum diameter ofcells ranging between 4.5 and 8.0 mm (mean�/6.3
mm, S.D.�/1.1 mm; N�/10).
Remarks. The microfossils described here under
Myxococcoides sp. resemble M. siderophila Gau-
cher (2000) in size, loose arrangement of cells in
irregular-shaped colonies and wall thickness.
Moreover, M. siderophila is a common component
of the Arroyo del Soldado microbiota. Never-theless, due to the advanced carbonization of the
material studied, an assignment to that taxon
would be premature.
4.3. Vendotaenids
Class Vendophyceae Gnilovskaya (1986)
Order Eoholyniales Gnilovskaya (1986)
Family Eoholyniaceae Gnilovskaya (1986)
Genus Eoholynia Gnilovskaya (1975)
Type species: Eoholynia mosquensis Gnilovskaya
(1975)
Eoholynia corumbensis sp. nov.
Fig. 7 A�/H
2000 Eoholynia sp. Gaucher: pl. 17.7
Type specimen . Holotype: specimen FC DP
3615 (Fig. 7C�/D). Paratypes of an individual
with multiple basal branchs is the specimen FC
DP 3222 (Fig. 7E, also figured by Gaucher, 2000:
pl. 17.7). As paratype of terminal sporangium, wedesignate the specimen FC DP 3616 (Fig. 7H).
Derivation of name. After the town of Corumba,
near which the fossils were discovered.
Material. Twenty five complete specimens plus
innumerable fragments preserved as carbonaceous
impressions in bedding surfaces of gray siltstones,
lowermost Guaicurus Formation.
Type locality. The material described here wascollected in the Pedreira Laginha, located along
road BR-262, 16 km south of Corumba (point 10,
Fig. 3). The fossils were found in situ in gray
siltstones with frequent slumps, 5�/10 m above the
contact with the Tamengo Formation (see strati-
graphic column in Gaucher, 2000: text*/fig. 39).
Diagnosis. A species of Eoholynia characterized
by a cord- or ribbon-like thallus composed of 3�/5main branches, 0.05�/0.6 mm wide (mean�/0.24
mm, S.D.�/0.15 mm, N�/34; Fig. 9). Main
branches develop secondary branches of several
orders, mostly dichotomously. Spherical bodies
are frequently (but not always) attached to the
surface of the main branch, and also to the ends of
the lateral branches (here interpreted as terminal
sporangia). Size of spherical bodies ranging be-tween 0.3 and 1.2 mm (mean�/0.54 mm, S.D.�/
0.26 mm, N�/11). Height of complete specimens
3�/18 mm, mostly around 8 mm.
Discussion. Eoholynia corumbensis differs from
Eoholynia mosquensis Gnilovskaya (1975) mainly
in its considerably larger size. While E. mosquensis
is characterized by main branches 0.1�/0.15 mm
wide (Gnilovskaya, 1979, 1985), E. corumbensis
reaches 0.6 mm (Fig. 9), thus four times wider than
the largest specimens of the former species. Side
branches of E. corumbensis are also much wider
and not so intensely tapering as that of E.
mosquensis . Finally, in the case of E. corumbensis ,
diameter of spherical bodies interpreted here as
sporangia is up to 10 times larger than that
reported for E. mosquensis (50�/100 mm: Gnilovs-kaya, 1979, 1985; Fig. 11). Considering the above
mentioned differences, we find that the erection of
a new species of the genus Eoholynia is well
justified.
Remarks. As already noted by Gnilovskaya
(1979, 1985) and Hofmann (1994), Eoholynia
represents the remains of eucaryotic algae, prob-
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 257
Page 18
ably Phaeophyta or Rhodophyta. Miaohephyton
bifurcatum Steiner (1994), a morphologically simi-
lar species, has been placed by Xiao et al. (1998)
among the brown algae (Phaeophyta). Neverthe-
less, the demonstration of the palaeobiologic
affinities of the species needs a more detailed
study that would be out of the scope of this paper.
E. corumbensis was probably benthic, as also
suggested for E. mosquensis (Gnilovskaya, 1985;
Burzin, 1996). The accumulations found in silt-
stones of the Guaicurus Formation are parau-
tochthonous, as is the case of most Russian
Fig. 7. Eoholynia corumbensis sp. nov. and Tawuia sp. from bedding surfaces of gray siltstones of the lowermost Guaicurus Formation
at Pedreira Laginha (point 10, Fig. 3). (A�/H) Eoholynia corumbensis sp. nov. (A) Complete and straight thallus (FCDP 3613) with
multiple branchs. (B) Specimen FCDP 3614 with three basal branchs and curious levogire arrangement. (C�/D) Holotype FCDP 3615,
with main branch covered with spherical bodies (sporangia). Terminal sporangia are present at the end of the main branch and at two
secondary branches. (E) Paratype FCDP 3222 with intensely branching thallus. (F) Straight thallus-fragment. (G) Terminal
sporangium. (H) Specimen FCDP 3616, paratype of terminal sporangium. (I�/J) Tawuia sp. (I) Slightly curved specimen FCDP 3617.
(J) Straight specimen with numerous superimposed Eoholynia -thalli. Scale bars represent 1 mm for all figures.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278258
Page 19
occurrences (Burzin, 1996). Regarding the strati-
graphic distribution and evolution of the genus
Eoholynia , the brazilian material is clearly younger
than E. mosquensis from the Redkino Series
(lower Valdaian) of the East European Platform,
because: (1) the lower Guaicurus Formation con-
cordantly overlies limestones of the uppermost
Tamengo Formation with 87Sr/86Sr isotopic rela-
tions of 0.7086 and d13C of up to �/5� PDB
(Boggiani, 1998; Fig. 12), which can be correlated
with upper Valdaian sections worldwide (Jacobsen
and Kaufman, 1999; Walter et al., 2000); and (2)
Gnilovskaya (1979, 1985) finds that Eoholynia is
typical of lower Valdaian rocks of the EastEuropean Platform, which are overlain by Vendo-
taenia -bearing rocks of the Kotlin Horizon (upper
Valdaian). As will be discussed later, Vendotaenia
occurs immediately below Eoholynia in the Cor-
umba Group (Zaine, 1991). In accordance with
our data, Hofmann (1992, 1994) reports younger
occurrences of Eoholynia , probably reaching the
Lower Cambrian.
It is worth noting that the larger size of E.
corumbensis compared with E. mosquensis could
reflect an evolutionary trend among the Eoholy-
niaceae. Older representatives of the family like the
Riphean genus Ulophyton are even smaller than E.
mosquensis (Gnilovskaya, 1979; Hofmann, 1994),
while younger representatives are mostly larger
(e.g. Steiner, 1994).
Order Vendotaeniales Gnilovskaya (1986)
Family Vendotaeniaceae Gnilovskaya (1986)Genus Vendotaenia Gnilovskaya (1971)
Type species: Vendotaenia antiqua Gnilovskaya
(1971)
Vendotaenia antiqua Gnilovskaya (1971)
Fig. 5A
1971 Vendotaenia antiqua Gnilovskaya: pl. XI.6-8
1979 Tyrasotaenia podolica Gnilovskaya
1985 Vendotaenia antiqua Gnilovskaya
1985 Tyrasotaenia podolica Gnilovskaya
1991 Tyrasotaenia sp. Zaine: fig. 5.6, pl. 10
1994 Vendotaenia antiqua Steiner: fig. 41k, l; fig. 45a�/c; fig. 47
Type specimen . Specimen IGGD AN SSSR, No.
6931/20 from the Kotlin Horizon in St. Petersburg,
figured by Gnilovskaya (1971: pl. XI.8).
Material . Zaine (1991) mentions the occurrence
of 25 measured specimens of Tyrasotaenia sp.
(synonimized by Steiner, 1994 with Vendotaenia )in marls of the Tamengo Formation. Our material
includes five fragments recovered by means of
palynological macerations from siltstones of the
Cerradinho Formation and phosphorites of the
Bocaina Formation (Fig. 5A).
Description . The material recovered from the
Cerradinho and Bocaina formations consists of
Fig. 8. Titanotheca coimbrae from phosphorites of the Bocaina
Formation at Fazenda Ressaca (point 6, Fig. 3). Specimens
shown in A�/C were recovered by means of standard palyno-
logical macerations, while specimen shown in D occurs in a
thin-section of the phosphorites.(A) Vase-shaped individual
with rather long neck. (B) Detail of A, showing broken part of
the test and agglutinated rutile-grains. (C) Vase-shaped speci-
men with large portions of test broken apart. Note septum
dividing initial chamber from neck (arrowed). (D) Large
specimen with broken apical end and coarse-grained wall. Scale
bars represent 25 mm for all specimens.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 259
Page 20
fragments of ribbonlike thalli, with typical long-
itudinal-fibrous structure due to compaction folds.
No cellular structure is present. Width of the
fragments reaches 0.12 mm. The fossils are carbo-
nized and degraded to some extent, showing gray
to black colour.Remarks . The fossils described by Zaine (1991)
as Tyrasotaenia sp. can be confidently assigned to
Vendotaenia antiqua . Steiner (1994) placed Tyra-
sotaenia in synonimy with Vendotaenia . More-
over, size and morphology of the specimens
illustrated by Zaine (1991) are well in accordance
with the diagnosis of Vendotaenia antiqua (Gni-
lovskaya, 1971, 1979, 1985; Steiner, 1994). The
fragmentary material recovered in this work from
the Cerradinho and Bocaina formations closely
resembles the microstructure of Vendotaenia anti-
qua (Gnilovskaya, 1985: pls. 32.3, 33.5, 34.3�/4;
Vidal, 1989: fig. 1C, E). Therefore, we assume that
they are conspecific with the vendotaenids of the
Tamengo Formation. It is worth noting that the
microstructure of Eoholynia from the Guaicurus
Formation is very different to that of Vendotaenia .
While the former represents eucaryotic, red or
Fig. 9. Size-frequency distribution of selected fossils from the Corumba and ASGs. Data shown for C. riemkeae of the ASG were
taken from Gaucher and Sprechmann (1999). Sources of histogrammes of C. lucianoi are Zaine (1991) (right graph) and this work (left
graph). Note important differences between the size-frequency distributions of this species, due to differential hydrodynamic sorting in
Cloudina -bearing tempestites. Size-frequency distribution of Eoholynia corumbensis n.sp. was measured at a single locality, where the
type material occurs.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278260
Page 21
brown algae, the latter are probably abandoned
sheaths of sulfide-oxidizing organotrophic bacteria
related to the Beggiatoaceae (Vidal, 1989),
although other interpretations cannot be ruled
out (Gnilovskaya, 1985).
Class Chuariaphyceae Gnilovskaya & Ishchenko,in Hofmann (1994)
Order Chuariales Gnilovskaya (1986)
Family Tawuiaceae Ishchenko, in Hofmann (1994)
Genus Tawuia Gnilovskaya (1975)
Fig. 10. C. lucianoi from thin-sections of carbonatic tempestites of the Tamengo Formation at Pedreira Corcal (point 8, Fig. 3). (A):
Longitudinal section of specimen with two stacked tubes. (B) Single, straight tube with closed apical end. (C) Specimen FCDP 3242
with same structure as C. riemkeae forma b Germs (1972). (D) Long, worm-like specimen (arrowed) with sinuous shell. Note
constriction at the middle of the test. (E) Two transverse sections. Note section with two eccentric tubes and robust spine (arrowed). (F)
Transverse section of two attached specimens. Note deformation of smaller specimen along attachment (arrowed). (G) Small specimen
with clear basal peduncule. (H�/I): Large, chalice-shaped specimen with robust spines (arrowed). Closed apical end of specimen is not
shown in H. (J): Chalice-shaped specimen with basal peduncule and two open ends. (K): Specimen FCDP 3241 with stacked tubes and
short, curved basal-peduncule. All scale bars represent 1 mm.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 261
Page 22
Type species: Tawuia dalensis Hofmann (1979, in
Steiner, 1994)
Tawuia sp.Fig. 7I�/J
Material. Five carbonaceous compressions in
bedding surfaces of gray siltsones of the Guaicurus
Formation at Pedreira Laginha (point 10, Fig. 3),
co-occuring with Eoholynia corumbensis .Description. Sausage-shaped carbonaceous
compressions, straight or ‘C’-shaped, with more
or less rounded ends. Width of compressions: 1.0�/
2.0 mm and length ranging between 5 and 12 mm.
Remarks. Most of the specimens observed are
either straight or slightly curved. Only one speci-
Fig. 11. C. riemkeae from thin-sections of banded siltstones of the Yerbal Formation (ASG). All specimens are preserved hematized
and filled with quartz and hematite. (A): Specimen FCDP 3001 showing: (1) short basal peduncule (?), and (2) two stacked cones
typical of the species. Compare with Fig. 10A. (B): Specimen FCDP 3236 with same morphology as C. lucianoi of Fig. 10K. (C)
Specimen FCDP 2934, showing bud with cone-in-cone structure (arrowed). (D) Specimen FCDP 2991, with same morphology as C.
riemkeae forma b Germs (1972). Compare also with Fig. 10C. (E) Specimen FCDP 3002, completely filled with hematite. (F)
Pedunculate specimen filled with hematite, closely parallelling the morphology of C. lucianoi shown in Fig. 10G. (G) Poorly preserved,
pedunculate specimen filled with sediment. (H) Long, worm-like specimen with sinuous test, analogous to individual shown in Fig.
10D. All scale bars represent 1 mm.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278262
Page 23
men shows the more typical C-shape. Steiner(1994: pl. 2) illustrates many specimens from the
Liulaobei Formation and Shilu Group of China
similar to those occuring in the Guaicurus For-
mation. The material described here fits well
within the size-range of Tawuia dalensis from
those units. Nonetheless, we leave these fossils in
open nomenclature until more material is avail-
able.
4.4. Skeletal fossils
Phylum indetClassis et ordo indet
Family Cloudinidae Hahn & Pflug (1985)
Genus Cloudina Germs (1972)
Type species. Cloudina hartmannae Germs (1972)
Cloudina lucianoi (Beurlen & Sommer) Zaine &
Fairchild (1985)Fig. 10A�/K
1957 Aulophycus lucianoi n. sp. Beurlen & Sommer: pls. I�/VI
1985 Cloudina lucianoi Zaine & Fairchild: p. 130
1985 Cloudina waldei Hahn & Pflug: fig. 7, pl. 1, pl. 2.3�/2.4
1987 Cloudina lucianoi Zaine & Fairchild: figs. 1�/7
1990 Cloudina lucianoi Grant: fig. 10E
1991 Cloudina lucianoi Zaine: figs. 5.2�/5.3, pls. 3�/6
1998 Cloudina lucianoi Boggiani: fig. 4.33
1999 Cloudina lucianoi Gaucher & Sprechmann: p. 66
2000 Cloudina lucianoi Gaucher: pl. 21.1�/21.3
Type specimen. Specimen No. 1149 of the
palaeobotanic collection of the Divisao de Geolo-gia e Mineralogia (DNPM, Rio de Janeiro),
figured by Beurlen and Sommer (1957: pl. II.b).
Material. Forty-five well-preserved specimens in
thin-sections of calcareous tempestites of the
Tamengo Formation at Pedreira Corcal (point 8,
Fig. 3), and many others in polished slabs of these
samples.
Dimensions. Maximum diameter of fossils stu-died ranging within 0.5�/3.5 mm (mean�/1.35 mm,
S.D.�/0.64 mm, N�/32), well in accordance with
earlier reports by Zaine (1991, 0.2�/3.8 mm) and
Hahn and Pflug (1985, 2.5�/3.1 mm). However,
size-frequency distribution of studied tafocenosis
(Fig. 9) varies according with competence of
palaeocurrents. Length of tubes reaches 11 mm,but Zaine (1991) describes specimens up to 15 mm
in length.
Description. Calcareous fossils consisting of
straight or curved tubes open at one end. Tubes
are vertically and eccentrically stacked one into
another giving cone-in-cone structures. Axes of
stacked tubes are often not parallel to each other.
A basal peduncule and robust spines are present insome individuals. Pedunculate specimens have
commonly two open ends (Fig. 7G, J�/K). Juxta-
posed specimens also occur (Fig. 7F). Shell
probably calcitic, but an important amount of
organic matter in the shell and incomplete miner-
alization is suggested by the flexibility observed in
some individuals, as noted by Grant (1990) for
Cloudina from the Nama Group.Discussion. Zaine (1991) considers that the
name Cloudina lucianoi (Beurlen and Sommer)
Zaine and Fairchild (1985) has priority over
Cloudina waldei Hahn and Pflug (1985) because
‘according to article 10.f of the International Code
of Zoological Nomenclature (1985), for an organ-
ism not as first classified as an animal but later so
classified, the original name is available if validunder the code that it was published. In this case
the genus Aulophycus must be abandoned, but the
specific name lucianoi is still valid’ (translation
from Zaine, 1991: 80). For these reasons, we
consider here C. lucianoi (Beurlen and Sommer)
Zaine and Fairchild (1985) as the valid name of
this species (see Ride et al., 1985). Therefore, C.
waldei Hahn and Pflug (1985) is to be consideredas a junior synonim of C. lucianoi , contrary to the
opinion of Conway Morris et al. (1990: 248).
The fossils represent benthic organisms which
lived in an upright position. Seilacher (1999)
interpret this organisms as (bio)mat-stickers rather
than sediment stickers, mainly because of their
small size compared with other well-known sedi-
ment stickers such as rugose corals. The inter-pretation of such a lifestyle for Cloudina , allowed
Seilacher (1999) to explain the anatomy of these
fossils in terms of functional morphology. Thus,
stacked cones of varying lengths can be explained
as organisms living in sedimentary environments
with different sedimentation rates, while common
kinks in the shell represent occasional perturba-
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 263
Page 24
tions. Therefore, the two forms (a and b) of C.
riemkeae described by Germs (1972) probably
represent two different ecophenotypes: forma arepresents specimens growing under low sedimen-
tation rate, while forma b grew under faster
accumulating sediment. The same is true for C.
lucianoi , which also shows specimens made up of
stacked cones of different lengths. C. lucianoi
presents other morphological variations, including
the presence of robust spines in 15% of the
observed individuals and 18% of pedunculate
specimens. As already mentioned by Hahn and
Pflug (1985: 425), C. lucianoi is larger than C.
riemkeae (diameter 0.3�/1.5 mm; Fig. 9) but
smaller than Cloudina hartmannae , (diameter
2.5�/6.5 mm: Germs, 1972).
Remarks. There are no substantial differences
between C. lucianoi of the Corumba Group and C.
riemkeae from the ASG other than their size-
frequency distribution (Gaucher and Sprechmann,
1999; Gaucher, 2000; Fig. 9) and preservation.
While specimens of C. riemkeae of the ASG are
completely hematized and preserved in life posi-
tion (autochthonous accumulation), C. lucianoi of
the Corumba Group is preserved in tempestites
(parautochthonous accumulation) with its origi-
nal, slightly recrystallized calcareous shell. There-
fore, it can be expected that C. lucianoi from
autochthonous accumulations would show a wider
size-frequency distribution than observed in our
material.
Order Foraminiferida Eichwald (1830)
Suborder Textulariina Delage & Herouard (1896)
Family Saccamminidae Brady (1884)
Subfamily Sacammininae Brady (1884)
Genus Titanotheca Gaucher & Sprechmann (1999)
Type species. Titanotheca coimbrae Gaucher &Sprechmann (1999)
Titanotheca coimbrae Gaucher & Sprechmann
(1999)
Figs. 8.A�/D
1999 Titanotheca coimbrae Gaucher & Sprechmann:
pls. 5�/6; pl. 7.1-7.2; text*/fig. 9
2000 Titanotheca coimbrae Gaucher: pl. 18.3�/18.7;
text*/fig. 34A�/F
Type specimen. Holotype: specimen figured by
Gaucher and Sprechmann (1999: pl. 5, figs. 1�/2)
with catalogue number FC DP 2947.
Material. Five complete specimens and many
fragments in thin-sections and acid macerates of
black phosphorites of the Bocaina Formation at
Fazenda Ressaca (point 6, Fig. 3).
Description. Spheroidal to vase-shaped fossils
with walls composed of a single layer of aggluti-nated rutile grains. Test is composed of one or two
chambers divided by a septum (Fig. 8C), and has
an open (apertural) end. Agglutinated rutile grains
are well sorted but vary considerably among
different specimens. While in the case of specimens
figured in Fig. 8A�/C long axes of rutile crystals
(3�/4 mm) are approximately 6% of maximum
diameter of the test, specimen shown in Fig. 8D
is composed of relatively larger crystals, with long
axes up to 14% of its maximum diameter (25 mm).
Maximum diameter of specimens ranging within
50�/175 mm. Largest specimen observed is 0.55 mmlong (Fig. 8D).
Remarks. On the basis of the agglutinated
nature of the wall, the simple shape of the fossils,
presence of perforated septa dividing the chambers
and small size, Gaucher and Sprechmann (1999)
assigned this genus to the Foraminiferida (sub-
order Textulariina), representing the oldest for-
aminifers currently known. The mode of life of
Titanotheca is inferred to have been free epi-
benthic, on the basis of the shape and robustness
of the test wall (Gaucher and Sprechmann, 1999;Gaucher, 2000). The specimens recovered from the
Bocaina Formation of the Corumba Group are the
only occurrence known to date other than the type
material from the ASG Group. The species has a
high fossilization potential and is quite abundant
in organic-rich facies such as the phosphorites of
the Bocaina Formation. If the taxon is confirmed
to be restricted to the upper Vendian (Valdaian) as
it seems, it may eventually become a useful index
fossil for that period.
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Page 25
5. Biostratigraphy
The palaeontologic data presented above for the
Corumba Group allows some important biostrati-
graphic correlations. First of all, the six genera of
organic-walled microfossils described from the
Corumba Group are also present in the ASG
(Gaucher, 2000). These genera are represented by
six species, three of which also occur in the ASG.The shared species, Bavlinella faveolata , Soldado-
phycus bossii and Leiosphaeridia tenuissima , are
the most abundant and dominate the microbiota
in both units. While in the ASG 19 species of
organic-walled microfossils have been described to
date (Gaucher, 2000), the diversity in the Corumba
Group is considerably lower. However, there is a
substantial preservational bias due to advancedcarbonization and degradation of fossils in the
latter unit. Most of the organic remains that were
found in macerations of the Corumba Group
could not be identified, for carbonization and
corrosion greatly obscures taxonomic-relevant de-
tails. The assemblages of the Corumba and ASGs
belong to the upper Vendian depauperate assem-
blage found in many successions worldwide(Chauvel and Schopf, 1978; Mansuy and Vidal,
1983; Volkova, 1985; Knoll and Sweet, 1985, 1987;
Germs et al., 1986; Palacios, 1989; Zaine, 1991;
Vidal et al., 1994; Gaucher et al., 1996). This
microbiota would match the Kotlin-Rovno assem-
blage of Vidal and Moczydlowska-Vidal (1997),
which is characterized by low diversity, abundance
of Bavlinella faveolata , rarity or absence ofacanthomorphs and absence of large sphaero-
morphs (Vidal and Knoll, 1983; Volkova, 1985;
Knoll, 1996a; Vidal and Moczydlowska-Vidal,
1997). More recently, another biostratigraphic
scheme has been proposed (Knoll, 2000 and
Walter et al., 2000), which recognizes three in-
formal biozones between the Varangerian/Mar-
inoan glacials and the base of the Cambrian. Theperiod immediately above the Varangerian is
characterized by a simple leiosphere palynoflora
(Walter et al., 2000: 383), followed by a complex
acanthomorph palynoflora described from many
sites worldwide (Knoll, 2000, and references
therein). In the uppermost Vendian (Kotlin-Rovno
assemblage of Vidal and Moczydlowska-Vidal,
1997), plankton diversity decreased dramatically,leading again to a depauperate assemblage domi-
nated by small sphaeromorphs. Although no
acanthomorphic acritarchs at all were found either
in the Corumba or ASG, the latter unit could be
divided into three informal zones according to
organic-walled microfossils. Gaucher (2000) re-
cognized two similar low-diversity assemblages
(Bavlinella -Soldadophycus assemblage) in the Yer-bal and Cerro Espuelitas Formations divided by a
more diverse assemblage occurring in the Polanco
Formation (Leiosphaeridia -Lophosphaeridium as-
semblage). These assemblages correlate chemos-
tratigraphically quite well with those proposed by
Knoll (2000) and Walter et al. (2000), the more
diverse assemblage occurring worldwide at a
positive d13C peak with 87Sr/86Sr values of0.7079, just as in the case of the Polanco Forma-
tion (Fig. 12). Gaucher (2000) proposed that this
alternation could be determined by different
palaeoclimatic conditions. Skeletal fossils strongly
indicate an upper Vendian age for most of both
units studied. Cloudina , for instance, has been
proposed by Grant (1990) as an index fossil of this
period, occurring in many successions worldwide(Germs, 1972; Palacios, 1989; Conway Morris et
al., 1990; Zaine, 1991; Bengtson and Yue Zhao,
1992; Vidal et al., 1994; Boggiani, 1998; Gaucher
et al., 1999; Gaucher, 2000; Hagadorn and Wagg-
oner, 2000; Corsetti and Hagadorn, 2000; Grot-
zinger et al., 2000). Therefore, the Corumba
Group is almost entirely upper Vendian in age,
for Cloudina occurs even at the top of theTamengo Formation (Boggiani, 1998). In the
case of the ASG, Cloudina indicates that the
Yerbal Formation was deposited in the upper
Vendian and not before (i.e. in the Varangerian).
On the other hand, Titanotheca seems to be
restricted to the lower parts of both groups
(Yerbal and Bocaina formations), co-occurring in
the ASG with Cloudina . This might imply that thestratigraphic range of Titanotheca is even more
restricted than Cloudina , but more work is needed
to prove this point.
Vendotaenids from the Guaicurus Formation
also give important biostratigraphic information.
Gnilovskaya (1979, 1985) finds that E. mosquensis
is typical of lower Valdaian rocks of the East
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 265
Page 26
Fig. 12. Chemostratigraphy of the Corumba and ASGs, compared with well studied sections from the Witvlei-Nama groups and the
Windermere Supergroup (Narbonne et al., 1994; Grotzinger et al., 1995; Kaufman et al., 1997; Saylor et al., 1998). Negative (N) and
positive (P) d13C-excursions were numbered starting from the first post-Varangerian/Marinoan deposits. Total thickness represents
approximately 2500 m for the Witvlei�/Nama groups (Saylor et al., 1998) and 5000 m for the Windermere Supergroup (Kaufman et
al., 1997). CSF: Cerros San Francisco Formation, CV: Cerro Victoria Formation, Blaube: Blaubeker Formation, Blass.: Blasskrans
Formation, Buschmannsk.: Buschmannsklippe Formation.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278266
Page 27
European Platform, which are overlain by Vendo-
taenia-bearing rocks of the Kotlin Horizon (upper
Valdaian). As already noted above, E. corumbensis
of the Guaicurus Formation postdates Vendotae-
nia -bearing marls of the Tamengo Formation, and
is thus younger than E. mosquensis . On the other
hand Vendotaenia antiqua is considered to be
restricted to the upper Vendian and occurs in
many successions worldwide. Some examples arethe East European Platform (Gnilovskaya, 1979,
1985; Burzin, 1996), China (Steiner, 1994 and
references therein) and Namibia (Germs et al.,
1986). The association of Vendotaenia with Ta-
wuia is known from the Dengying Formation of
China (Steiner, 1994), of uppermost Vendian age.
Finally, it has been suggested that Corumbella
werneri Hahn et al. (1982) from the CorumbaGroup (Tamengo Formation) could be a repre-
sentative of the Ediacara fauna (Zaine,
1991Walde, 1987: 103) or even belong to the
sabelleditids (Fairchild et al., 2000). In either
case, these fossils would indicate an upper Vendian
(Ediacaran, Valdaian) age for the Tamengo For-
mation. However, an alternative interpretation of
Corumbella as scyphozoans (Cnidaria) cannot beruled out (Hahn et al., 1982; Hagadorn and
Waggoner, 2000), though it seems rather improb-
able (Zaine, 1991).
Summing up, there is quite a lot of biostrati-
graphic information that points unequivocally to
an upper Vendian (Valdaian) age for both groups.
Furthermore, the striking similarity of the biotas
that lived in both basins suggests that they had anample connection or were even part of the same
shelf. It is worth noting that not only the more
widespread planktonic genera and species are
shared, but also representatives of the benthos
like Cloudina and Titanotheca .
6. Chemostratigraphy
The first stable isotope analyses for the Cor-
umba Group were reported by Zaine (1991), but
the data were unfortunately not ordered against
their stratigraphic position, and are of very limited
value for correlation. Boggiani (1998) reports
detailed carbon, oxygen and strontium isotope
analyses for the Corumba and ASGs, among otherNeoproterozoic-Cambrian units of South Amer-
ica. Gaucher (1999, 2000) presents a curve of d13C
variations in carbonates of the ASG, based on the
data reported by Boggiani (1998). Finally, Kawa-
shita et al. (1999) report d13C, d18O and 87Sr/86Sr
data for four samples from the base of the Polanco
Formation of the ASG. The stratigraphic rele-
vance of these determinations have been discussedby Gaucher et al. (1999). We present here new
d13C and d18O determinations for 13 samples of
the upper Yerbal Formation and lowermost Bar-
riga Negra Formation of the ASG. The database
includes, for the Corumba Group, 37 d13C and
d18O determinations, while in the case of the ASG,
d13C and d18O was determined for 30 samples.
Raw data are given in Table 1, while syntheticd13C curves for the Corumba and ASGs are shown
in Fig. 12. Furthermore, preliminary data of a
high-resolution chemostratigraphic survey of the
Polanco Formation (Gaucher et al., 2002) were
also considered. As for Sr-isotopes, a series of 2087Sr/86Sr determinations for carbonates of the
Corumba Group was carried out by Zaine
(1991). Boggiani (1998) presents 6 87Sr/86Sr ana-lyses for the Tamengo Formation. Finally, Kawa-
shita et al. (1999) report the only 87Sr/86Sr data
available for the ASG, consisting of four determi-
nations in little altered samples (Mn/Sr�/0.1; d18O
PDB�/�/11�) of the lowermost Polanco Forma-
tion.
6.1. Corumba Group
The comparison of the d13C curve of the
Corumba Group with the Vendian global curve
of Kaufman and Knoll (1995) and Kaufman et al.
(1997), Saylor et al. (1998), Jacobsen and Kauf-
man (1999) and Knoll (2000); Walter et al. (2000)
shows many significant coincidences. Dolomites of
the lowermost Bocaina Formation capping glacial
deposits of the Puga Formation yielded d13Cvalues of �/5.5 � PDB (Boggiani, 1998), a feature
typical of cap dolomites worldwide (Aitken, 1991;
Narbonne et al., 1994; Kennedy, 1996; Hoffman et
al., 1998; Knoll, 2000; Walter et al., 2000).
Dolomites of the Bocaina Formation up section
have d13C values near zero (Boggiani, 1998), while
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 267
Page 28
Table 1
Carbon and oxygen isotopic data of carbonates of the ASG
Locality Sample d18OSMOW� d18OPDB� d13CPDB� Precision (d18O) Precision (d13C) Remarks
ILL 2 ILL2A �/24.986 �/5.698 �/3.484 0.028 0.007 Stromatolitic limestone, Cerro Victoria Formation
ILL 4 ILL 4 �/24.989 �/5.695 �/0.174 �/ �/ Stromatolitic limestone, Cerro Victoria Formation
ILL 4 ILL 4B �/24.279 �/6.384 �/1.840 0.013 0.006 Stromatolitic limestone, Cerro Victoria Formation
PRJ 37 980317/5 �/20.254 �/10.288 �/1.692 0.003 0.005 Calcarenite, basal B. Negra Fm
PRJ 37 980317/7A �/20.514 �/10.036 �/1.738 0.005 0.014 Lim/dol rhytmite, calcarenite layer, boundary Polanco-B. Negra Fm
PRJ 37 980317/7B �/21.464 �/9.115 �/1.440 0.006 0.005 Lim/dol rhytmite, dolosiltite layer, boundary Polanco-B. Negra Fm
PRJ 13 PRJ 13C �/16.870 �/13.571 �/2.411 0.011 0.006 Limestone, upper Polanco Fm
PRJ 13 PRJ 13P �/16.122 �/14.297 �/2.597 0.006 0.003 Limestone, upper Polanco Fm
PRJ 14 PRJ 14B �/14.798 �/15.581 �/1.998 0.010 0.004 Limestone, upper Polanco Fm
PRJ 14 PRJ 14a1 �/16.726 �/13.711 �/2.808 0.008 0.003 Limestone, upper Polanco Fm
PRJ 14 PRJ 14 b2 �/17.551 �/12.911 �/2.888 0.005 0.002 Limestone; upper Polanco Fm
CPA 35 CPA 35 �/20.892 �/9.670 �/3.630 0.044 0.011 Lim/dol rhymite, C.Espuelitas Fm
CPA 32 970320/7 �/16.000 �/14.415 �/1.331 0.004 0.005 Lim/dol rhymite, C. Espuelitas Fm
CPA 31 970320/6 �/17.445 �/13.013 �/2.516 0.004 0.012 Limestone, basal C. Espuelitas Fm
CPA 30* 000715/3a �/18.139 �/12.341 �/1.944 0.008 0.011 Lim/dol rhytimite, uppermost Polanco Formation
CPA 30* 000715/3b �/20.589 �/9.963 �/1.547 0.005 0.008 Lim/dol rhytmite, uppermost Polanco Fm
CPA 30* 000715/4 �/21.982 �/8.612 �/1.783 0.003 0.012 Dolosiltite, upper Polanco Fm
CPA 40 970503/6 �/14.248 �/16.115 �/1.397 0.004 0.007 Lim/dol rhytmite, Polanco Fm
CPA 28 CPA28D �/19.328 �/11.187 �/2.596 0.019 0.009 Lim/dol rhytmite, lower PolancoFm
CPA 27 CPA 28C �/21.169 �/9.400 �/1.143 �/ �/ Lim/dol rhytmite, lower PolancoFm
CPA 27 CPA 28B �/15.719 �/14.688 �/1.391 0.007 0.004 Lim/dol rhytmite, lower PolancoFm
CPA 27 CPA 28A �/22.902 �/7.720 �/0.891 0.005 0.006 Lim/dol rhytmite, lower PolancoFm
PYE 31 980305/1 �/ �/12.3 �/2.82 �/ �/ Limestone, lowermost Polanco Fm
PYE 32 980305/2 �/ �/11.0 �/2.86 �/ �/ Limestone, lowermost Polanco Fm
CCU 4 980306/12 �/ �/11.4 �/2.56 �/ �/ Limestone, lowermost Polanco Fm
CCU 4 980306/13 �/ �/11.3 �/2.60 �/ �/ Limestone, lowermost Polanco Fm
MIN 15 99027/1 �/26.237 �/4.484 �/2.154 0.005 0.005 Pink dolosiltite, upper Yerbal Fm
MIN 15 990927/2a �/26.185 �/4.535 �/1.553 0.003 0.003 Pink dolosiltitle, upper Yerbal Fm
MIN 16 980311/6 �/26.957 �/3.786 �/1.957 0.002 0.009 Dolosiltite clast, upper Yerbal Fm
MIN 16 980927/1 �/25.174 �/5.516 �/1.167 0.006 0.009 Pink dolosiltite, upper Yerbal Fm
Data from localities PYE 31-32 and CCU 4 are from Kawashita et al. (1999). Analysis from localities CPA 27-28, PRJ 13-14 and ILL 2, 4 were reported by Boggiani
(1998). Other localities: this work.
C.
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esearch
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Page 29
limestones of the lower Tamengo Formationrecord a negative d13C excursion (�/2.5 to �/
3.3� PDB) associated with Corumbella - and
Cloudina -bearing strata (Boggiani, 1998). The
negative peaks of the middle Sheepbed Formation
(Windermere Supergroup) and the upper Witvlei-
lower Nama Group (N2: Fig. 12) resemble values
obtained for the lower Tamengo Formation (Nar-
bonne et al., 1994; Kaufman et al., 1997; Saylor etal., 1998). Moreover, this negative peak is globally
associated with the first occurrence of relatively
diverse Ediacaran fossils and Cloudina (Narbonne
et al., 1994: 1284; Saylor et al., 1998: 1233), as in
the Corumba Group. Up section in the Tamengo
Formation, a positive d13C excursion of up to �/
5.8� PDB is recorded in Cloudina -rich beds,
which has been correlated by Boggiani et al.(1996, 1997), with the younger Ediacaran (upper
Vendian) positive excursion (P2, Fig. 12) recorded
worldwide. Uppermost Tamengo limestones show
also positive d13C values, and are thus older than
the Gametrail-Zaris negative excursion N3 (Fig.
12). This important negative excursion is probably
represented in the lower Guaicurus Formation,
but no isotopic data for this unit are available.Alternatively, it could be correlated to the small
d13C-negative peak in the uppermost Tamengo
Formation, with 87Sr/86Sr values of 0.7086 (Bog-
giani, 1998).
6.2. Arroyo del Soldado Group
Varangerian�/Marinoan deposits are not known
from the ASG. Since the lower Yerbal Formationis devoid of carbonates, the post-Varanger nega-
tive excursion (N1, Fig. 12) is not recorded in the
ASG. Pink dolostones of the upper Yerbal For-
mation show positive d13C values of up to �/2.2�PDB (Table 1, Fig. 12), which continue into the
lowermost Polanco Formation, reaching �/2.9�PDB (Kawashita et al., 1999). This peak corre-
sponds to the positive d13C-peak of the lowerSheepbed Formation (Narbonne et al., 1994;
Kaufman et al., 1997) and middle Witvlei Group
(Saylor et al., 1998), because: (a) it is upper
Vendian (post-Varangerian) in age, as demon-
strated by C. riemkeae occurring in the uppermost
Yerbal Formation; and (b) it includes limestones
with 87Sr/86Sr values of 0.7078 (Kawashita et al.,
1999), similar to those encountered in the lower
Sheepbed Formation (Windermere Supergroup)
and in the Buschmannsklippe Formation of the
Witvlei Group (Narbonne et al., 1994; Kaufman et
al., 1997; Walter et al., 2000: 376). Although the
remarkably thick, pure and little altered carbo-
nates of the overlying Polanco Formation repre-
sent the desideratum for the determination of a
precise d13C curve, reconnaissance studies carried
out by Boggiani (1998) and Gaucher (2000) and
the authors yielded ambiguous results. The stra-
totype of the Formation, located in the Tapes
Grande Syncline (Gaucher, 2000) has shown
exclusively negative values troughout the section
(sites CPA 27 to CPA 30* of Table 1). Never-
theless, other sections in the Isla Patrulla Block
and Piraraja region (sites CCU 4, PYE 31�/32 and
PRJ 13�/14 of Table 1; Gaucher et al., 2002) show
that the lower and upper Polanco Formation
records positive d13C excursions, and that carbo-
nates there experienced little diagenetic overprint
(Mn/Sr mostly within 0.05 and 0.5; d18O between
�/6 and �/11�). Differences in the carbon iso-
topic composition could respond to an enhanced
metamorphic overprint at the stratotype, or more
probably, to a very characteristic, deeper sedimen-
tary environment in which primary dolomite
precipitation (Vasconcelos and McKenzie, 1997;
Burns et al., 2000) due to bacterial sulphate
reduction took place (Gaucher, 2000: 32). In
such disaerobic environments, recycling of organic
matter by bacteria yields isotopically light CO2,
which is then incorporated into carbonates, result-
ing in 13C-depletion (Strauss et al., 1992; Kaufman
and Knoll, 1995: 30; Hoefs, 1997: 122). Calver
(2000) proposed that 13C-depletion of deep-water,
upper Vendian carbonates of the Adelaide Rift
Complex (Australia) responds to water stratifica-
tion, which also played a key role in the ASG
(Gaucher, 2000). The sections in which positive
values have been recorded are characterized by
lower palaeobathymetry, above storm-weather
wave-base, and also by less dolomite precipitation.
In modern marine environments, shallow seawater
is enriched in 13C by approximately 1� PDB
relative to deep water (Hoefs, 1997: 121). Ongoing
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 269
Page 30
research with detailed sampling of sections up to800 m thick is intended to solve this problem.
All things considered, it is reasonably well
established that two positive d13C excursions are
recorded in the lowermost Polanco (P1, Fig. 12)
and upper Polanco Formation (P2), separated by
an abrupt negative excursion to �/3.5� PDB (N2).
The transition from the Polanco Formation into
the Cerro Espuelitas Formation or the lowermostBarriga Negra Formation is marked by a shift to
negative values of �/1.7� PDB (lowermost Bar-
riga Negra Formation) or �/3.6� PDB (lower
Cerro Espuelitas Formation). This negative peak
is tentatively correlated to 13C-depleted carbonates
of the N3 excursion (Fig. 12) recorded in the
Gametrail Formation (Windermere Supergroup;
Kaufman et al., 1997) and uppermost ZarisFormation (Nama Group; Grotzinger et al.,
1995; Saylor et al., 1998). No carbonates are
known from the upper Cerro Espuelitas or Cerros
San Francisco Formation, the youngest record
being stromatolitic and oolitic limestones of the
Cerro Victoria Formation (Montana and Sprech-
mann, 1993; Gaucher, 2000). Three analyses
reported by Boggiani (1998) suggest the presenceof a further negative d13C excursion. Taking into
account the lowermost Cambrian age proposed for
this unit on the basis of trace fossils (Montana and
Sprechmann, 1993; Gaucher, 2000), the mentioned
negative peak could match the negative d13C
excursion of the Proterozoic-Cambrian boundary
N4 (Fig. 12) recorded worldwide (Kaufman et al.,
1997; Saylor et al., 1998; Knoll, 2000; Corsetti andHagadorn, 2000), but more data are needed to
confirm this.
Summing up, there is a reasonably good corre-
lation between the d13C curves obtained for the
Corumba and ASGs with the global curve of the
upper Vendian, thus supporting other lines of
evidence such as biostratigraphy and geochronol-
ogy. The available 87Sr/86Sr data for the Corumbaand ASGs also corroborate an upper Vendian age
for these units (Kaufman et al., 1993; Gorokov et
al., 1995; Kaufman et al., 1997; Knoll, 2000;
Walter et al., 2000). Sedimentation begun already
in the Varangerian in the Corumba basin (Puga
Formation), unlike the ASG, which begins with
post-glacial eustatic sea-level rise. On the other
hand, d13C-data suggest that deposition of theASG probably continued into the lowermost
Cambrian, unlike the Corumba Group. We are
confident that high-resolution carbon chemostra-
tigraphy will allow to refine these correlations
(Gaucher et al., 2002).
7. Discussion
7.1. Palaeogeography
All lines of evidence presented above indicate
that: (1) the Arroyo del Soldado and Corumba
groups are coeval; (2) broadly the same climatic
and eustatic sea-level changes are recorded in both
units; and (3) both basins had either an ample
connection or were part of the same shelf.Furthermore, the units studied here were clearly
deposited in a passive, Atlantic-type continental
margin (Almeida, 1984: 274; Zaine, 1991; Boggiani
et al., 1993; Gaucher et al., 1996, 1998a,b;
Boggiani, 1998; Gaucher 1999, 2000). In both
groups, the platform deepened to the east, with
granitic-metamorphic source areas located to the
west (Boggiani, 1990, 1998; Boggiani et al., 1993;Montana and Sprechmann, 1993; Gaucher, 2000).
Considering the above mentioned facts, the only
plausible explanation is that the Corumba and
ASGs were deposited onto the same shelf. This
hypotesis is supported by the same tectonic
vergence to the W-NW and same structural style
(Almeida, 1984: 269; Boggiani et al., 1993; Gau-
cher et al., 1998b; Gaucher, 2000; Alvarenga et al.,2000: 187), as well as similar ages for the deforma-
tion of these units around 540�/490 Ma (see
chapter 2). Furthermore, there are many outcrops
of probably correlative successions in northern
Uruguay, Paraguay and Bolivia, scattered in a
roughly NS direction between the main outcrop
areas (Fig. 13). Finally, the nature of the basement
is also similar in both cases. This suggests thateither the Rio de la Plata Superterrane (Gaucher,
2000) extends further to the north than previously
thought (Almeida et al., 1973) or that the Rıo de la
Plata Superterrane and the Amazonian Craton
were already amalgamated into a single crustal
block by Vendian times (Fig. 13).
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278270
Page 31
The possibility that the Rıo de la Plata Super-
terrane (Craton) could extend to the north to
latitudes around 208S has been already suggested
by Ramos (1988). While studying a basement
window in southern Paraguay (Fig. 13), Meinhold
(1998) and Meinhold et al. (2000) also postulated
the continuity of the Rio de la Plata Craton to the
north. Ramos (1988) suggested that the collision of
the Rıo de la Plata Craton with the ensialic Alto
Paraguay Terrane, located to the east, resulted in
deformation of the Paraguay belt. The Alto
Paraguay Terrane of Ramos (1988) is roughly
equivalent to the Parana (Continental) Block of
Soares (1988, Fig. 13), and has been considered in
the palaeogeographic reconstructions of Unrug
(1996). The study of this crustal block is hindered
by the sedimentary cover of the Parana Basin
(Figs. 1 and 13), but some data has been obtained
by interpretation of gravimetric and magneto-
metric surveys, as well as datings of basement
Fig. 13. Major cratonic areas in SE-South America, showing extension of the Corumba-Arroyo del Soldado palaeoshelf. Major
lineaments and boundary of post-Cambrian cover were modified from Almeida et al. (1973). Probable boundaries of the Parana Block
according with Soares (1988). Western boundary of the Rıo de la Plata Superterrane taken from Ramos (1988). Position of the
hypothetical Mesoproterozoic suture between the Amazonian Craton and the Rıo de la Plata Superterrane is not known due to
extensive vulcanosedimentary cover of the Parana Basin. DFB, Dom Feliciano Belt; RB, Ribeira Belt.
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 271
Page 32
rocks from oil exploration drilling (Ramos, 1988;Soares, 1988). It follows from this discussion that
the collision of the Rıo de la Plata Superterrane
with the Parana Block (Alto Paraguay Terrane) in
the Cambrian is responsible for the closure and
deformation of the Arroyo del Soldado-Corumba
shelf. The deformed sedimentary cover marks the
suture between this two continental blocks (Fig.
13). A similar palaeogeographic reconstruction hasbeen recently presented by Teixeira (2000), while
studying upper Vendian�/Cambrian (‘Eocam-
brian’) sedimentary successions from SE-Brazil.
The author suggested that closure of the Brazilides
ocean between the Amazonian-Rio de la Plata
cratons and the Parana Block-Sao Francisco
Craton resulted in deformation of the Corumba
shelf in the Cambrian. Brito Neves et al. (1999)and Campos Neto (2000) proposed a slightly
different palaeogeographic scheme for the region.
7.2. Palaeoclimatology
Drastic climatic changes took place during
deposition of the Corumba and ASGs (Section
3.2). Climate shifted quite rapidly between ‘green-
house’ and ‘icehouse’ effects, as known fromsedimentary successions of this age worldwide
(Kasting, 1992; Kirschivink, 1992). Gaucher
(2000) proposed that these changes were driven
mainly by palaeoceanographic factors that
strongly altered bioproductivity. Kaufman (2000)
arrived to a similar model through other lines of
evidence. The model proposed by Gaucher (2000)
includes the following sequence of events:(1) In the Upper Riphean, enhanced hydroter-
mal activity (Knoll, 1994) related to the rifting of
Rodinia (Hoffman, 1991; Dalziel, 1995; Unrug,
1996) pump large quantities of iron, silica, and
other nutrients into the ocean. These accumulate
in the deep ocean, generating an enormous reser-
voir, leading to ocean stratification.
(2) Due to a favourable wind and currentregime, enhanced upwelling takes place. Upwelling
zones represent only 0.1% of the present ocean
surface (Baturin, 1982), but this need not be
constant through geologic time. Enhanced upwel-
ling of nutrient-rich waters stored in the deep-
ocean reservoir trigger massive phytoplankton
blooms (Palacios, 1989; Vidal and Nystuen,1990), deposition of BIF and phosphorites. The
ocean becomes largely eutrophic, and large
amounts of organic matter accumulate on the
shelves. Sulphate-reducing bacteria find optimal
conditions in the anoxic lower water-layer, leading
to anomalous sulphate consumption and shift in
d34S towards extremely positive values (Walter et
al., 2000: 415; Kaufman, 2000).(3) Drawdown of CO2 by blooming phytoplank-
ton-populations reduces greenhouse effect and
triggers glaciation (Kaufman et al., 1997; Kauf-
man, 2000) and eustatic sea-level fall. Concomi-
tantly, large amounts of oxygen would be released
into the atmosphere, accounting for the proposed
Neoproterozoic oxygen-increase (Holland, 1994;
Canfield and Teske, 1996).(4) During snowball earth, silicate weathering,
carbonate deposition and bioproductivity are
greatly reduced, allowing for accumulation of
atmospheric CO2.
(5) Restored greenhouse effect leads to global
warming, ice melting and sea-level rise. Consump-
tion of nutrients stored in deep waters, a reduction
of hydrotermal activity after the Varangerian (asshown by 87Sr/86Sr relations: Knoll, 1994; Jacob-
sen and Kaufman, 1999; Walter et al., 2000) and
better water mixing seem to be the cause for the
final disruption of the system in the Early Cam-
brian.
7.3. Palaeobiology
As already proposed by Grant (1990) andGaucher and Sprechmann (1999), the fact that
shelly fossils were so widespread and relatively
diverse in the upper Vendian suggests that biomi-
neralization and agglutination were not simply a
local phenomenon by latest Proterozoic times.
Grotzinger et al. (2000) describe the calcareous
fossil Namacalathus hermanastes from reefs of the
Nama Group, adding one more species to theknown diversity of Precambrian shelly fossils.
They further report the occurrence of undeter-
mined, tube-shaped, calcareous fossils from the
same reefs. The Nama Group of Namibia and the
ASG of Uruguay are thus the units which contain
the most diverse shelly assemblages reported to
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278272
Page 33
date (Germs, 1972; Gaucher and Sprechmann,1999; Gaucher, 2000; Grotzinger et al., 2000).
This suggests that favourable palaeogeographic
and consequently palaeoclimatic conditions in that
regions (see above) stimulated the diversification
of mineralized fossils. Considering that both
basins were probably located at either sides of
the same ocean (Unrug, 1996; Teixeira, 2000), it is
probable that the ‘oasis’ generated in that wayrepresents in fact the craddle of many different
lineages, which could evolve despite the inhospi-
table conditions elsewhere. Furthermore, the di-
versity of types of biomineralization and
agglutination found in the upper Vendian is
surprising, suggesting that the advent of skeletons
must have occurred much earlier than currently
accepted.
8. Conclusions
The Arroyo del Soldado and Corumba groups
are roughly coeval, and were deposited onto the
same passive continental margin, developed on the
Rıo de la Plata Superterrane. Thus, this continen-
tal block probably extends farther to the norththan previously expected, or, alternatively, it was
already amalgamated to the Amazonian Craton by
Vendian times. Closure of the basin is related to
the collision of the platform with the Parana Block
(Alto Paraguay Terrane) during the Cambrian-
Early Ordovician. Broadly the same climatic and
eustatic sea-level changes are recorded in both
units. Climate shifted quite rapidly between ‘green-house’ and ‘icehouse’ effects, with the consequent
lithological variations. Varangerian/Marinoan gla-
ciogenic deposits are not known from the Arroyo
del Soldado basin, but are represented in the
Corumba basin by the Puga Formation. This
suggests that the ASG was deposited in a lower
palaeolatitude, inside the circum-equatorial (B/
258), ice-free region predicted by recent computersimulations of Neoproterozoic glaciations. New
data obtained further support recent models of
Neoproterozoic climate change and glaciations
proposed independently by Gaucher (2000) and
Kaufman (2000). These models are based on
enhanced bioproductivity driven by high nutrient
availability in the largely stratified Neoproterozoicoceans. Newly discovered fossils are described
from the Corumba Group, with a total of six
species of organic-walled microfossils, three spe-
cies of vendotaenids and two species of skeletal
fossils including Cloudina and Titanotheca . The
vendotaenid Eoholynia corumbensis sp. nov is
described from siltstones of the Guaicurus For-
mation. The biota preserved in the CorumbaGroup correlates well with that represented in
the ASG. An upper Vendian (Valdaian) age is
confirmed for both units, using biostratigraphic,
as well as carbon and strontium isotopic data.
While uppermost deposits of the ASG are of
lowermost Cambrian age, sedimentation ceased
already in the uppermost Vendian in the Corumba
basin. Nevertheless, continuing research in thepoorly known, uppermost units of the Corumba
Group (Guaicurus Formation) might reveal the
Proterozoic/Cambrian transition as well. Finally,
an important diversity of skeletal fossils in the
Corumba, Arroyo del Soldado and Nama groups
points to favourable palaeoclimatic conditions in
the region occupied by the ocean that extended
between the Rıo de la Plata and Kalahari cratons.It is here proposed that this favourable conditions
generated an ‘oasis’, in which skeletal fossils and
metazoans could evolve despite the inhospitable
conditions elsewhere.
Acknowledgements
This study is the product of fruitful cooperationbetween Brazilian and Uruguayan researchers.
The authors are indebted to many individuals
who collaborated in many respects. Deep gratitude
is due to the late Armando Marcio Coimbra
(1949�/1998), who actively participated of field
work and contributed with stimulating ideas, but
could not see the completion of this publication.
Ana Lucia Desenzi Gesicki (Campo Grande) andSilvana Martınez (Montevideo) are thanked for
valuable help during field work in Brazil and
Uruguay, respectively. Antonio Luiz Teixeira
(Sao Paulo) is thanked for providing literature
and for valuable comments. Our sincere apprecia-
tion goes to Prof. Malcom Walter and an anon-
C. Gaucher et al. / Precambrian Research 120 (2003) 241�/278 273
Page 34
ymous reviewer for their useful comments andsuggestions. Field work in Uruguay has been
financed by research project 1040 of the Consejo
Nacional de Investigacion Cientıfica y Tecnologica
(CONICYT), Uruguay. Analyses and publication
expenses were financed by research projects C-32
and C-39 of the Comision Sectorial de Investiga-
cion Cientıfica (CSIC, Uruguay) and project
CONICYT 6007. C.G. is indebted to the GermanAcademic Exchange Service (DAAD) for a gener-
ous research grant (1995�/1999) and further finan-
cial support in many respects. This paper is a
contribution to projects IGCP 450 (Proterozoic
sediment-hosted base metal deposits of western
Gondwana) and IGCP 419 (Foreland basins of the
Neoproterozoic belts in central to southern Africa
and South America).
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